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$ar = array (

  array("level" => '1', "caption" => "一级菜单1"),

  array("level" => '1', "caption" => "一级菜单2"),

  array("level" => '2', "caption" => "二级菜单1"),

  array("level" => '2', "caption" => "二级菜单2"),

  array("level" => '3', "caption" => "三级菜单1"),

  array("level" => '1', "caption" => "一级菜单3"),

);

print_r( foo($ar));



function foo(&$ar, $level='1') {

  $res = array();

  while($r = current($ar)) {

    if($r['level'] > $level) $res[count($res)-1]['sub'] = foo($ar, $r['level']);

    elseif($r['level'] == $level) $res[] = $r;

    else {

      array_unshift($ar, $r);

      break;

    }

    array_shift($ar);

  }

  return $res;

}