diff --git a/config/set/downgrade-php80.php b/config/set/downgrade-php80.php
index ce89592e..9e13c73c 100644
--- a/config/set/downgrade-php80.php
+++ b/config/set/downgrade-php80.php
@@ -23,6 +23,7 @@
 use Rector\DowngradePhp80\Rector\FuncCall\DowngradeStrContainsRector;
 use Rector\DowngradePhp80\Rector\FuncCall\DowngradeStrEndsWithRector;
 use Rector\DowngradePhp80\Rector\FuncCall\DowngradeStrStartsWithRector;
+use Rector\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector;
 use Rector\DowngradePhp80\Rector\FunctionLike\DowngradeMixedTypeDeclarationRector;
 use Rector\DowngradePhp80\Rector\FunctionLike\DowngradeUnionTypeDeclarationRector;
 use Rector\DowngradePhp80\Rector\Instanceof_\DowngradeInstanceofStringableRector;
@@ -91,5 +92,6 @@
         RemoveReturnTypeDeclarationFromCloneRector::class,
         DowngradeEnumToConstantListClassRector::class,
         DowngradeInstanceofStringableRector::class,
+        DowngradeSubstrFalsyRector::class,
     ]);
 };
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/DowngradeSubstrFalsyRectorTest.php b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/DowngradeSubstrFalsyRectorTest.php
new file mode 100644
index 00000000..e7f4d292
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/DowngradeSubstrFalsyRectorTest.php
@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector;
+
+use Iterator;
+use PHPUnit\Framework\Attributes\DataProvider;
+use Rector\Testing\PHPUnit\AbstractRectorTestCase;
+
+final class DowngradeSubstrFalsyRectorTest extends AbstractRectorTestCase
+{
+    #[DataProvider('provideData')]
+    public function test(string $filePath): void
+    {
+        $this->doTestFile($filePath);
+    }
+
+    public static function provideData(): Iterator
+    {
+        return self::yieldFilesFromDirectory(__DIR__ . '/Fixture');
+    }
+
+    public function provideConfigFilePath(): string
+    {
+        return __DIR__ . '/config/configured_rule.php';
+    }
+}
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/by_variable_string.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/by_variable_string.php.inc
new file mode 100644
index 00000000..464b3fc6
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/by_variable_string.php.inc
@@ -0,0 +1,27 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+class ByVariableString
+{
+    public function run(string $name)
+    {
+        return substr($name, 2);
+    }
+}
+
+?>
+-----
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+class ByVariableString
+{
+    public function run(string $name)
+    {
+        return (string) substr($name, 2);
+    }
+}
+
+?>
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/fixture.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/fixture.php.inc
new file mode 100644
index 00000000..ea7b6685
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/fixture.php.inc
@@ -0,0 +1,27 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+class Fixture
+{
+    public function run()
+    {
+        return substr("a", 2);
+    }
+}
+
+?>
+-----
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+class Fixture
+{
+    public function run()
+    {
+        return (string) substr("a", 2);
+    }
+}
+
+?>
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_already_casted.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_already_casted.php.inc
new file mode 100644
index 00000000..bd271390
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_already_casted.php.inc
@@ -0,0 +1,11 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+class SkipAlreadyCasted
+{
+    public function run()
+    {
+        return (string) substr("a", 2);
+    }
+}
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_append.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_append.php.inc
new file mode 100644
index 00000000..bdde85fa
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_append.php.inc
@@ -0,0 +1,13 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+class SkipAppend
+{
+    public function run(string $a)
+    {
+        $a .=substr('a', 2);
+
+        return $a;
+    }
+}
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_as_array_dim_fetch_dim.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_as_array_dim_fetch_dim.php.inc
new file mode 100644
index 00000000..7e2d68fa
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_as_array_dim_fetch_dim.php.inc
@@ -0,0 +1,12 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+class SkipAsArrayDimFetchDim
+{
+    public function run()
+    {
+        $data[substr('a', 2)] = 'foo';
+        var_dump($data);
+    }
+}
\ No newline at end of file
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_as_array_key.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_as_array_key.php.inc
new file mode 100644
index 00000000..2e98fcdd
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_as_array_key.php.inc
@@ -0,0 +1,15 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+class SkipAsArrayKey
+{
+    public function run()
+    {
+         $data = [
+            substr('a', 2) => 'foo'
+        ];
+
+        var_dump($data);
+    }
+}
\ No newline at end of file
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_as_falsy_arg.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_as_falsy_arg.php.inc
new file mode 100644
index 00000000..50c8efb3
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_as_falsy_arg.php.inc
@@ -0,0 +1,16 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+class SkipAsFalsyArg
+{
+    public function run()
+    {
+        return $this->execute(substr('a', 2));
+    }
+
+    private function execute(false|string $value)
+    {
+        return $value;
+    }
+}
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_casted_bool.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_casted_bool.php.inc
new file mode 100644
index 00000000..f78c7eeb
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_casted_bool.php.inc
@@ -0,0 +1,11 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+class SkipCastedBool
+{
+    public function run(string $name)
+    {
+        return (bool) substr($name, 2);
+    }
+}
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_compared_to_false.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_compared_to_false.php.inc
new file mode 100644
index 00000000..619faa6a
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_compared_to_false.php.inc
@@ -0,0 +1,11 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+class SkipComparedToFalse
+{
+    public function run(string $name)
+    {
+        substr($name, 2) === false;
+    }
+}
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_different_func_call.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_different_func_call.php.inc
new file mode 100644
index 00000000..449c45f2
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_different_func_call.php.inc
@@ -0,0 +1,11 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+class SkipDifferentFuncCall
+{
+    public function run()
+    {
+        return strlen('a');
+    }
+}
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_direct_cond.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_direct_cond.php.inc
new file mode 100644
index 00000000..a1998c9f
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_direct_cond.php.inc
@@ -0,0 +1,13 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+class SkipDirectCond
+{
+    public function run(string $name)
+    {
+        if (substr($name, 2)) {
+            return true;
+        }
+    }
+}
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_empty.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_empty.php.inc
new file mode 100644
index 00000000..9c8b0476
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_empty.php.inc
@@ -0,0 +1,14 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+/**
+ * empty can be false or empty string ""
+ */
+class SkipEmpty
+{
+    public function run()
+    {
+        return empty(substr("a", 2));
+    }
+}
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_in_binary_op.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_in_binary_op.php.inc
new file mode 100644
index 00000000..73392e4a
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_in_binary_op.php.inc
@@ -0,0 +1,11 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+class SkipInBinaryOp
+{
+    public function run($a)
+    {
+        return substr("a", 2) && $a === true;
+    }
+}
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_negated.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_negated.php.inc
new file mode 100644
index 00000000..ed83c522
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_negated.php.inc
@@ -0,0 +1,14 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+/**
+ * negation can be false or empty string ""
+ */
+class SkipNegated
+{
+    public function run()
+    {
+        return ! substr('a', 2);
+    }
+}
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_negative_offset.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_negative_offset.php.inc
new file mode 100644
index 00000000..9a942831
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_negative_offset.php.inc
@@ -0,0 +1,14 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+/**
+ * zero offset with no length can't be false
+ */
+class SkipNegativeOffset
+{
+    public function run(string $name)
+    {
+        return substr($name, -1);
+    }
+}
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_never_empty.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_never_empty.php.inc
new file mode 100644
index 00000000..b9936e86
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_never_empty.php.inc
@@ -0,0 +1,11 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+class SkipNeverEmpty
+{
+    public function run()
+    {
+        return substr("abc", 2);
+    }
+}
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_new_arg_falsy.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_new_arg_falsy.php.inc
new file mode 100644
index 00000000..d464729d
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_new_arg_falsy.php.inc
@@ -0,0 +1,15 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+class SkipNewArgFalsy
+{
+    public function __construct(false|string $value)
+    {
+    }
+
+    public function run()
+    {
+        new self(substr('a', 2));
+    }
+}
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_sprintf.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_sprintf.php.inc
new file mode 100644
index 00000000..851758d6
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_sprintf.php.inc
@@ -0,0 +1,18 @@
+<?php
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+/**
+ * sprintf %s format converts false to empty string
+ */
+class SkipSprintf
+{
+    public function run(string $name)
+    {
+        return sprintf('Value: %s', substr($name, 2));
+    }
+
+    public function run2(string $name)
+    {
+        printf('Value: %s', substr($name, 2));
+    }
+}
\ No newline at end of file
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_ternary_falsy.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_ternary_falsy.php.inc
new file mode 100644
index 00000000..c0bffd12
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_ternary_falsy.php.inc
@@ -0,0 +1,11 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+class SkipTernaryFalsy
+{
+    public function run()
+    {
+        return substr('a', 2) ?: 'default';
+    }
+}
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_with_concat.php.inc.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_with_concat.php.inc.inc
new file mode 100644
index 00000000..b7aa24db
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_with_concat.php.inc.inc
@@ -0,0 +1,14 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+/**
+ * falsy concat become empty string
+ */
+class SkipWithConcat
+{
+    public function run()
+    {
+        return substr('a', 2) . substr('b', 2);
+    }
+}
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_zero_offset_with_no_length.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_zero_offset_with_no_length.php.inc
new file mode 100644
index 00000000..f05440b4
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_zero_offset_with_no_length.php.inc
@@ -0,0 +1,14 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+/**
+ * zero offset with no length can't be false
+ */
+class SkipZeroOffsetWithNoLength
+{
+    public function run(string $name)
+    {
+        return substr($name, 0);
+    }
+}
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_zero_offset_with_positive_length.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_zero_offset_with_positive_length.php.inc
new file mode 100644
index 00000000..cfead068
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/skip_zero_offset_with_positive_length.php.inc
@@ -0,0 +1,14 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+/**
+ * zero offset with postive length can't be false
+ */
+class SkipZeroOffsetWithPositiveLength
+{
+    public function run(string $name)
+    {
+        return substr($name, 0, 10);
+    }
+}
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/with_zero_offset_length_negative.php.inc b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/with_zero_offset_length_negative.php.inc
new file mode 100644
index 00000000..1119a039
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/Fixture/with_zero_offset_length_negative.php.inc
@@ -0,0 +1,33 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+/**
+ * @see https://3v4l.org/lv13M#v7.4.33
+ */
+class WithZeroOffsetLengthNegative
+{
+    public function run(string $name)
+    {
+        return substr($name, 0, -5);
+    }
+}
+
+?>
+-----
+<?php
+
+namespace Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\Fixture;
+
+/**
+ * @see https://3v4l.org/lv13M#v7.4.33
+ */
+class WithZeroOffsetLengthNegative
+{
+    public function run(string $name)
+    {
+        return (string) substr($name, 0, -5);
+    }
+}
+
+?>
diff --git a/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/config/configured_rule.php b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/config/configured_rule.php
new file mode 100644
index 00000000..1f861b32
--- /dev/null
+++ b/rules-tests/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector/config/configured_rule.php
@@ -0,0 +1,10 @@
+<?php
+
+declare(strict_types=1);
+
+use Rector\Config\RectorConfig;
+use Rector\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector;
+
+return static function (RectorConfig $rectorConfig): void {
+    $rectorConfig->rule(DowngradeSubstrFalsyRector::class);
+};
diff --git a/rules/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector.php b/rules/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector.php
new file mode 100644
index 00000000..9c20aeed
--- /dev/null
+++ b/rules/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector.php
@@ -0,0 +1,218 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Rector\DowngradePhp80\Rector\FuncCall;
+
+use PhpParser\Node;
+use PhpParser\Node\Arg;
+use PhpParser\Node\ArrayItem;
+use PhpParser\Node\Expr;
+use PhpParser\Node\Expr\ArrayDimFetch;
+use PhpParser\Node\Expr\AssignOp;
+use PhpParser\Node\Expr\BinaryOp;
+use PhpParser\Node\Expr\BinaryOp\Concat;
+use PhpParser\Node\Expr\BinaryOp\Identical;
+use PhpParser\Node\Expr\BooleanNot;
+use PhpParser\Node\Expr\CallLike;
+use PhpParser\Node\Expr\Cast;
+use PhpParser\Node\Expr\Cast\String_;
+use PhpParser\Node\Expr\Empty_;
+use PhpParser\Node\Expr\FuncCall;
+use PhpParser\Node\Expr\MethodCall;
+use PhpParser\Node\Expr\New_;
+use PhpParser\Node\Expr\StaticCall;
+use PhpParser\Node\Expr\Ternary;
+use PhpParser\Node\Stmt\Do_;
+use PhpParser\Node\Stmt\If_;
+use PhpParser\Node\Stmt\While_;
+use PHPStan\Reflection\FunctionReflection;
+use PHPStan\Reflection\MethodReflection;
+use PHPStan\Type\Constant\ConstantIntegerType;
+use Rector\NodeTypeResolver\PHPStan\ParametersAcceptorSelectorVariantsWrapper;
+use Rector\PhpParser\Node\Value\ValueResolver;
+use Rector\PHPStan\ScopeFetcher;
+use Rector\Rector\AbstractRector;
+use Rector\Reflection\ReflectionResolver;
+use Symplify\RuleDocGenerator\ValueObject\CodeSample\CodeSample;
+use Symplify\RuleDocGenerator\ValueObject\RuleDefinition;
+
+/**
+ * @changelog https://php.watch/versions/8.0/substr-out-of-bounds
+ *
+ * @see \Rector\Tests\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector\DowngradeSubstrFalsyRectorTest
+ */
+final class DowngradeSubstrFalsyRector extends AbstractRector
+{
+    /**
+     * @var string
+     */
+    private const IS_FALSY_UNCASTABLE = 'is_falsy_uncastable';
+
+    public function __construct(
+        private readonly ReflectionResolver $reflectionResolver,
+        private readonly ValueResolver $valueResolver
+    ) {
+
+    }
+
+    public function getRuleDefinition(): RuleDefinition
+    {
+        return new RuleDefinition('Downgrade substr() with cast string on possibly falsy result', [
+            new CodeSample('substr("a", 2);', '(string) substr("a", 2);'),
+        ]);
+    }
+
+    /**
+     * @return array<class-string<Node>>
+     */
+    public function getNodeTypes(): array
+    {
+        return [
+            Cast::class,
+            Empty_::class,
+            BooleanNot::class,
+            Ternary::class,
+            Identical::class,
+            Concat::class,
+            MethodCall::class,
+            StaticCall::class,
+            New_::class,
+            AssignOp::class,
+            If_::class,
+            While_::class,
+            Do_::class,
+            ArrayItem::class,
+            ArrayDimFetch::class,
+            BinaryOp::class,
+            FuncCall::class,
+        ];
+    }
+
+    /**
+     * @param Cast|Empty_|BooleanNot|Ternary|Identical|Concat|MethodCall|StaticCall|New_|AssignOp|If_|While_|Do_|ArrayItem|ArrayDimFetch|BinaryOp|FuncCall $node
+     */
+    public function refactor(Node $node): ?Node
+    {
+        if ($node instanceof Cast || $node instanceof Empty_ || $node instanceof BooleanNot || $node instanceof AssignOp) {
+            $node->expr->setAttribute(self::IS_FALSY_UNCASTABLE, true);
+            return null;
+        }
+
+        if ($node instanceof Ternary) {
+            if (! $node->if instanceof Expr) {
+                $node->cond->setAttribute(self::IS_FALSY_UNCASTABLE, true);
+            }
+
+            return null;
+        }
+
+        if ($node instanceof Concat) {
+            $node->left->setAttribute(self::IS_FALSY_UNCASTABLE, true);
+            $node->right->setAttribute(self::IS_FALSY_UNCASTABLE, true);
+            return null;
+        }
+
+        if ($node instanceof Identical) {
+            if ($this->valueResolver->isFalse($node->left)) {
+                $node->right->setAttribute(self::IS_FALSY_UNCASTABLE, true);
+            }
+
+            if ($this->valueResolver->isFalse($node->right)) {
+                $node->left->setAttribute(self::IS_FALSY_UNCASTABLE, true);
+            }
+
+            return null;
+        }
+
+        if ($node instanceof If_ || $node instanceof While_ || $node instanceof Do_) {
+            $node->cond->setAttribute(self::IS_FALSY_UNCASTABLE, true);
+            return null;
+        }
+
+        if ($node instanceof ArrayItem) {
+            if ($node->key instanceof Expr) {
+                $node->key->setAttribute(self::IS_FALSY_UNCASTABLE, true);
+            }
+
+            return null;
+        }
+
+        if ($node instanceof ArrayDimFetch) {
+            if ($node->dim instanceof Expr) {
+                $node->dim->setAttribute(self::IS_FALSY_UNCASTABLE, true);
+            }
+
+            return null;
+        }
+
+        if ($node instanceof BinaryOp) {
+            $node->left->setAttribute(self::IS_FALSY_UNCASTABLE, true);
+            $node->right->setAttribute(self::IS_FALSY_UNCASTABLE, true);
+            return null;
+        }
+
+        if ($node instanceof CallLike) {
+            if ($node->isFirstClassCallable()) {
+                return null;
+            }
+
+            $reflection = $this->reflectionResolver->resolveFunctionLikeReflectionFromCall($node);
+
+            if (! $reflection instanceof MethodReflection && ! $reflection instanceof FunctionReflection) {
+                return null;
+            }
+
+            $parameterAcceptor = ParametersAcceptorSelectorVariantsWrapper::select(
+                $reflection,
+                $node,
+                ScopeFetcher::fetch($node)
+            );
+
+            foreach ($parameterAcceptor->getParameters() as $position => $parameterReflection) {
+                if ($parameterReflection->getType()->isFalse()->no()) {
+                    continue;
+                }
+
+                $arg = $node->getArg($parameterReflection->getName(), $position);
+                if ($arg instanceof Arg) {
+                    $arg->value->setAttribute(self::IS_FALSY_UNCASTABLE, true);
+                }
+            }
+        }
+
+        if (! $this->isName($node, 'substr')) {
+            return null;
+        }
+
+        if ($node->getAttribute(self::IS_FALSY_UNCASTABLE) === true) {
+            return null;
+        }
+
+        $type = $this->getType($node);
+        if ($type->isNonEmptyString()->yes()) {
+            return null;
+        }
+
+        $offset = $node->getArg('offset', 1);
+
+        if ($offset instanceof Arg) {
+            $offsetType = $this->getType($offset->value);
+            if ($offsetType instanceof ConstantIntegerType && $offsetType->getValue() <= 0) {
+                $length = $node->getArg('length', 2);
+                if ($length instanceof Arg) {
+                    $lengthType = $this->getType($length->value);
+                    if ($lengthType instanceof ConstantIntegerType && $lengthType->getValue() >= 0) {
+                        return null;
+                    }
+
+                    return new String_($node);
+                }
+
+                return null;
+            }
+        }
+
+        return new String_($node);
+    }
+}
diff --git a/tests/Issues/DowngradeHeredoc/Fixture/next_arg.php.inc b/tests/Issues/DowngradeHeredoc/Fixture/next_arg.php.inc
index 1f117df8..db204c12 100644
--- a/tests/Issues/DowngradeHeredoc/Fixture/next_arg.php.inc
+++ b/tests/Issues/DowngradeHeredoc/Fixture/next_arg.php.inc
@@ -474,9 +474,9 @@ final class NextArg
         $lineString = trim($lineString);
 
         if (strncmp($lineString, '/**', strlen('/**')) === 0) {
-            $lineString = substr($lineString, 3);
+            $lineString = (string) substr($lineString, 3);
         } elseif (strncmp($lineString, '*', strlen('*')) === 0) {
-            $lineString = substr($lineString, 1);
+            $lineString = (string) substr($lineString, 1);
         }
 
         return trim($lineString);