array (size=2)
0 =>
array (size=1)
'order_id' => '868' (length=3)
1 =>
array (size=1)
'order_id' => '871' (length=3)
$ids = array_reduce($order_id, function($ids, $value){ return array_merge($ids, array_values($value)); }, array());
array (size=2)
0 => '868' (length=3)
1 => '871' (length=3)
如果第二維是數(shù)字鍵名
那么直接這樣就可以了：
$result = array_reduce($user, 'array_merge', array())
