strrev

<?php
/**
 * Reverse a miltibyte string.
 *
 * @param string $string The string to be reversed.
 * @param string|null $encoding The character encoding. If it is omitted, the internal character encoding value
 *     will be used.
 * @return string The reversed string
 */
function mb_strrev(string $string, string $encoding = null): string
{
    $chars = mb_str_split($string, 1, $encoding ?: mb_internal_encoding());
    return implode('', array_reverse($chars));
}
?>
It's faster and flexible than tianyiw function (comment #122953)

This function support utf-8 encoding, Human Language and Character Encoding Support:

<?php
function mb_strrev($str){
    $r = '';
    for ($i = mb_strlen($str); $i>=0; $i--) {
        $r .= mb_substr($str, $i, 1);
    }
    return $r;
}

echo mb_strrev("☆❤world"); // echo "dlrow❤☆"
?>