Clean implementation of the Luhn algorithm in PHP

luhn.php

<?php

/** Validates a credit card number using the Luhn algorithm.
 *  http://en.wikipedia.org/wiki/Luhn_algorithm
 *
 *  @param string $card_number The card number (without spaces, dots, or other
 *  	special characters, only the numbers).
 *
 *  @return boolean True if the number is valid, false otherwise.
 *
 */

public function is_card_number_valid($card_number)
{
	$digits = str_split($card_number);
	$n = count($digits);

	$sum = 0;
	$dbl = false;

	for($i = $n - 1; $i >= 0; $i--)
	{
		$digit = $digits[$i];

		if($dbl)
		{
			$digit = $digit * 2;
			if($digit >= 10) $digit -= 9;
		}

		$dbl = !$dbl;
		$sum += $digit;
	}

	if($sum % 10 == 0)
		return true;

	return false;
}

A lot of Luhn Algorithm implementations on PHP around the net are unnecessarily complex, they do operations over the array keys or strange, inefficient ways of adding the numbers of the partial products. Here is a simple, and cleaner implementation.

Tested with credit card numbers generated with: http://www.getcreditcardnumbers.com/