Simple Luhn formula function

gistfile1.php

<?php
/* From http://adrianotto.com/2011/10/better-luhn-formula-validator-for-php/ */
/*
 *   Copyright 2011 Adrian Otto
 *
 *   Licensed under the Apache License, Version 2.0 (the "License");
 *   you may not use this file except in compliance with the License.
 *   You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 *   Unless required by applicable law or agreed to in writing, software
 *   distributed under the License is distributed on an "AS IS" BASIS,
 *   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *   See the License for the specific language governing permissions and
 *   limitations under the License.
 */

function luhn_validate($s) {
    if (0 == $s) {  // Don't allow all zeros
        return(false);
    }

    $sum = 0;
    $i = strlen($s);     // Find the last character
    while ($i-- > 0) {   // Iterate all digits backwards
        $sum += $s[$i];  // Add the current digit

        if ($i % 2 != 0) {
            continue;
        }

        // If the digit is even, add it again.
        // Adjust for digits 10+ by subtracting 9.
        if ($s[$i] > 4) {
            $sum += $s[$i] - 9;
        }
        else {
            $sum += $s[$i];
        }
    }

    return ($sum % 10 == 0);
}

?>