Check if a string is palindrome

is_palindrome.php

<?php

// <!-- $string = "A man, a plan, a canal, Panama"; -->

function is_palindrome($string)
{
    $a = strtolower(preg_replace("/[^A-Za-z0-9]/", "", $string));
    return $a == strrev($a);
}

var_dump(is_palindrome('racecar')); // true
var_dump(is_palindrome('foobar'));  // false
var_dump(is_palindrome('noon'));    // true