How to reverse a string

Dear friend,
Today, almost all modern languages have their own built-in methods to reverse a string. For example, PHP have strrev($stringtoreverse) while ruby has stringtoreverse.reverse utility to reverse the string. Availability of these methods prohibit us to think about the small-small logic behind these methods. Sometimes, it may be the harmful for developer community too. Following is the code in PHP to reverse a string:

<?php

    echo str_rev('abcxyz'); //called the method

    function str_rev($stringtoreverse) //strrev is built-in method. So, str_rev is defined as custom function

    {

        $revstr=''; //initially empty string

        $len=strlen($stringtoreverse);

        for($i=$len-1;$i>=0;$i--)

        {

            $revstr.=substr($stringtoreverse,$i,1); //concat each character returned by substr method

        }

        return $revstr;

    }  

?>

Please feel free to make your valuable comment.