Interview Challenge: Write a function to test if a string is a palindrome

I had an interview yesterday for a Senior Software Developer role. I can't remember the question very well, but I try to write it as much as I remember.

Question:

Write a function that takes a string as the input and return true if traversed, false if not.

Requirements: (as much as I remember)

• There is only one input type: string.
• The string parameter must be passed by reference.
• No new variables should be declared within the function -- he meant that if the parameter is called $str, then manipulation should only be done on that. He didn't like me using a second variable to set $reverseStr etc.
• No looping every character in the string, he specifically said not to loop thru every character in the string.
• No built-in PHP functions could be used; I think he was fine with strlen().

My Answer: (nope he didn't like it)

$str = 'this is testing';
$length = strlen($str);
$reverseStr = '';
for($i=$length-1; $i>=0; $i--) {
 $reverseStr .= $str[$i];
}

While this does the job somewhat, he didn't like me to go through every character to get $reverseStr. I am guessing I needed to think about a recursive solution get the last character of the string and index etc., but I am thinking about these as I am writing this, too late!

What you guys thinking?

the question isn't clear at all.. but if you want to check if the string is a palindrome:

function is_palindrome ($str){
    if(strlen($str)==0){return true;}
    if($str[0]==$str[strlen($str)-1]){
        return true and is_palindrome(substr($str, 1, strlen($str)-2));
    }else{
        return false;
    }
}