php多维数组合并成一个数组,该如何处理

$a = Array(
  Array( 'id' => 1 ),
  Array( 'id' => 2 )
);

$a = array_map('current', $a);
print_r($a); 

Array
(
    [0] => 1
    [1] => 2
)

$a = Array(
  Array( 'id' => 1 ),
  Array( 'id' => 2 )
);
$ar = array();
foreach($a as $v) $ar[] = $v['id'];
print_r($ar);