<?php

	function subtrai_array( $arr1, $arr2 ) {
		foreach ( $arr2 as $chave => $valor ) {
		    if( array_key_exists( $chave, $arr1 ) ) {
		        $arr1[$chave] = $arr1[$chave] - $valor;
			} else {
				$arr1[$chave] = -$valor;
			}
		}
		return $arr1;
	}

	$a1 = Array (
	    'bife' => 3,
	    'batata' => 2,
	    'arroz' => 1,
		'zóião' => 3
	);
	$a2 = Array (
	    'bife' => 2,
	    'batata' => 2,
	    'couve' => 1
	);
	print_r( subtrai_array( $a1, $a2 ) );
?>