add string reverse test

Author: kevindees

.gitignore

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+.idea

str-reverse/array.php

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+<?php
+list($script, $option, $n) = $argv;
+$s = microtime(true);
+
+function dispatch($str) {
+    return implode(array_reverse(mb_str_split($str)));
+}
+
+$range = range(1,  $n ?? 1);
+foreach ($range as $i) {
+    dispatch($option);
+}
+
+echo ( microtime(true) - $s) * 1000;

str-reverse/readme.md

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+A not-so-scientific `implode > array_reverse > mb_str_split` vs `for loop with mb_substr` speed test.
+
+Run Commands:
+
+```
+// args: string "starting with a", "num times to run"
+php substr.php abcd 10000
+php array.php abcd 10000
+```
+
+### Huge Set ~1_000_000 calls
+
+- *Array*: `401.384ms`
+- *SubStr*: `999.577ms`
+
+### Large Set ~10000 calls
+
+- *Array*: `5.398ms`
+- *SubStr*: `12.624ms`
+
+### Small Set ~100 calls
+
+- *Array*: `0.070ms`
+- *SubStr*: `0.147ms`
+
+My conclusion is that `implode >array_reverse > mb_str_split` is much better if detecting the first char.

str-reverse/substr.php

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+<?php
+list($script, $option, $n) = $argv;
+$s = microtime(true);
+
+function dispatch($str) {
+    $r = '';
+    for ($i = mb_strlen($str); $i>=0; $i--) {
+        $r .= mb_substr($str, $i, 1);
+    }
+    return $r;
+}
+
+$range = range(1,  $n ?? 1);
+foreach ($range as $i) {
+    dispatch($option);
+}
+
+echo ( microtime(true) - $s) * 1000;