Search for most cost effective package combination (shipping)

shipping.php

<?php

function fill_packages( $packages ) {
	$overrun    = 42; // Items in cart
	$cost       = 0;
	$filled     = array();
	$quantities = array();
	foreach( $packages as $the_package ){
		$delivers = floor( $overrun / $the_package['contain'] );
		if( ! $delivers ) {
			continue;
		}
		$filled[$the_package['name']] = $delivers;
		$overrun = $overrun - ( $delivers * $the_package['contain'] );
		$cost = $cost + ( $delivers * $the_package['cost'] );
	}
	if( $overrun ) {
		return 'Error, Overrun of ' . $overrun . '!';
	}
	return array(
		'packages' => $filled,
		'cost'     => $cost,
	);
}

/* Test it! */

print_r(
	fill_packages(
		array(
			array(
				'name' => '18er',
				'contain' => 18,
				'cost' => 5
			),
			array(
				'name' => '12er',
				'contain' => 12,
				'cost' => 5
			),
			array(
				'name' => '6er',
				'contain' => 6,
				'cost' => 8
			),
		)
	)
);