PHP 数组合并的有关问题

PHP 数组合并的问题
我有两个数组，分别是这样的：
Array ( [0] => 1 [1] => 1 [2] => 1 [3] => 1 [4] => 1 )
Array ( [0] => 4 [1] => 2 [2] => 5 [3] => 6 [4] => 7 )
我想把这两个数组合并，变成如下的新数组：
Array ( [1] => 4 [1] => 2 [1] => 5 [1] => 6 [1] => 7 )
但是，我用array_combine，但是合并以后的数组变成了Array ( [1] => 7 )
应该要如何操作啊？

$a = Array ( 0 => 1, 1 => 1, 2 => 1, 3 => 1, 4 => 2, 5 => 2, 6 => 2 ); 

$b = Array ( 0 => 7, 1 => 2, 2 => 5, 3 => 3, 4 => 3, 5 => 1, 6 => 5 );


$result = array();

foreach($a as $k => $v) {

	if(!isset($result[$v])) {

		$result[$v] = array(); 

	}


	$result[$v][] = $b[$k];

}


print_r($result);