How to check if the given string is palindrome? [closed]

PHP sample:

$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);
}

Removes any non-alphanumeric characters (spaces, commas, exclamation points, etc.) to allow for full sentences as above, as well as simple words.