如何通过php将一维数组转换为3个连续值组成的二维数组？

<?php
$aaa = array('aa','bb','cc','dd','ee','ff','gg','hh','ii');
for($i=0;$i<3;$i++)
{
 $bbb[] = array_slice($aaa, $i * 3 ,3);
}
print_r($bbb);
?>

Array
(
 [0] => Array
  (
   [0] => aa
   [1] => bb
   [2] => cc
  )
 [1] => Array
  (
   [0] => dd
   [1] => ee
   [2] => ff
  )
 [2] => Array
  (
   [0] => gg
   [1] => hh
   [2] => ii
  )
)

$bbb[] = array_slice($aaa, $i * 3 ,3);
//3为3个一组，如果是2为2个一组