原数组

$a = array ();

$b = array (
'id' => 1,
'pid' => 0,
'sotr' => 0
);
$c =array (
'id' => 2,
'pid' => 1,
'sotr' => 0
);
$d = array (
'id' => 3,
'pid' => 1,
'sotr' => 1
);
想要要得到的数组

array(
'0'=>array ( 'id' => 1, 'pid' => 0, 'sotr' => 0),
'1'=>array ( 'id' => 2, 'pid' => 1, 'sotr' => 0),
'2'=>array ( 'id' => 3, 'pid' => 1, 'sotr' => 1)
);

需要什么什么方法合并呢？