number_tower.php

<?php
/**
 * Number Tower
 */

$rows = array(
	array( 21, 24, 0, 9, 0, 0, 2 ),
	array( 45, 30, 0, 11, 9, 0 ),
	array( 75, 0, 0, 20, 0 ),
	array( 0, 0, 0, 38 ),
	array( 0, 117, 0 ),
	array( 0, 201 ),
	array( 0 ),
);

foreach( $rows as $index => $row ) {
	foreach ( $row as $key => $value ) {
		if ( $value == 0 && $index == 0 ) {
			$rows[ $index ][ $key ] = $rows[ $index + 1 ][ $key - 1 ] - $rows[ $index ][ $key - 1 ];
		} elseif ( $value == 0 && $index > 0 ) {
			$rows[ $index ][ $key ] = $rows[ $index - 1 ][ $key ] + $rows[ $index - 1 ][ $key + 1 ];
		}
	}
}

echo '<pre>';
var_dump( $rows );
echo '</pre>';