<?php
function  combine($array,$start,$key,$newkey){
  static $new; //静态变量
  foreach($array as $k=>$v){
    if($v[$key]==$start){
      $new[$v[$newkey]][] = $v;
      unset($array[$k]);
      continue;
    }
  }
  sort($array);
  if(count($array)!==0){
    combine($array,$array[0][$key],$key,$newkey);//递归调用
  }
  return $new;
}

$array=array();
$array=array(array('a'=>'aaa','b'=>'bbb'),array('a'=>'aaa','b'=>'ccc'),array('a'=>'123','b'=>'3456'));
$key='a';
$newkey='a';
$list=combine($array,$array[0][$key],$key,$newkey);
foreach($list as $key=>$val){
    $new_list[]=$val;
}
print_r($new_list);
/**
 * Array
(
    [0] => Array
        (
            [0] => Array
                (
                    [a] => aaa
                    [b] => bbb
                )

            [1] => Array
                (
                    [a] => aaa
                    [b] => ccc
                )

        )

    [1] => Array
        (
            [0] => Array
                (
                    [a] => 123
                    [b] => 3456
                )

        )

)

 */
?>