<?php
$str = "ABCDEFG";
echo strrev($str);

<?php
$str = "ABC DEF";
$len = strlen($str);
$new_str = '';
while ($len) {
    $new_str .= $str[$len - 1];
    $len--;
}
echo $new_str;

<?php
    $str = 'hello world 你好';
    $len = mb_strlen($str);
    $result = '';
    for ($i = $len-1; $i>=0; $i--){
        $result.= mb_substr($str,$i,1,'utf-8');
    }
    echo $result;

<?php
function recursion ($str) {
    if (strlen($str) > 0) {
        recursion(substr($str, 1));
    }
    echo substr($str, 0, 1);
    return;
}
$str = "ABCDEFG";
recursion($str);