PHP::Reverse string in a recursive way

Sometimes I feel baffled when I am asked some kind of algorithmic questions. I think I need to keep the mindset of algorithms. That is my take away today.

<?php
// Reverse string in a recursive way
/* This is a general question, I believe. */
function getRevString($str, $count = 0) {
    $length = strlen($str);
    if ( $count < ($length/2) ) {
        // swapping characters according the string index
        --$length;
        $temp = $str[$count];
        $str[$count] = $str[$length-$count];
        $str[$length-$count] = $temp;

        // don't forget "return" statement in order to get the result
        return getRevString($str, ++$count);
    }
    return $str;
}

// testing the function
$str = "Hello World";
$test = getRevString($str);
echo "Result::" . $test;