encode_ean13.php

<?php

function encode_ean13($number)
{
	// String it and make it the right length
	$digits = (string)$number;
	$digits = strlen($digits) < 12 ? str_pad($digits, 12, '0', STR_PAD_LEFT) : $digits;

	// Add the values of the digits in the even-numbered positions: 2, 4, 6, etc.
	$even_sum = $digits{1} + $digits{3} + $digits{5} + $digits{7} + $digits{9} + $digits{11};

	// Multiply this result by 3.
	$even_sum_three = $even_sum * 3;

	// Add the values of the digits in the odd-numbered positions: 1, 3, 5, etc.
	$odd_sum = $digits{0} + $digits{2} + $digits{4} + $digits{6} + $digits{8} + $digits{10};

	// Sum the results of steps 2 and 3.
	$total_sum = $even_sum_three + $odd_sum;

	// The check character is the smallest number which, when added to the result in step 4,  produces a multiple of 10.
	$next_ten = (ceil($total_sum/10))*10;
	$check_digit = $next_ten - $total_sum;

	return $digits.$check_digit;
}

// EOF