PHP数组中如果有一个元素相同，怎么拼接其他元素

PHP 数组中如果有一个元素相同，如何拼接其他元素
比如原数组是这样的
array(3) {
[0] => array(6) {
["pla_id"] => string(3) "298"
["user_id"] => string(1) "1"
["due_in"] => string(15) "8.33"
}
[1] => array(6) {
["pla_id"] => string(3) "976"
["user_id"] => string(1) "1"
["due_in"] => string(15) "338.90"
}
[2] => array(6) {
["pla_id"] => string(3) "56"
["user_id"] => string(1) "2"
["due_in"] => string(15) "25.6"
}}
因为数组中的user_id=1 是一样的
要得到最后这样数组，要怎么写(就是把user_id相同的其他元素拼接在一起)？
array(3) {
[0] => array(6) {
["pla_id"] => string(3) "298,976"
["user_id"] => string(1) "1"
["due_in"] => string(15) "8.33,338.90"
}
[1] => array(6) {
["pla_id"] => string(3) "56"
["user_id"] => string(1) "2"
["due_in"] => string(15) "25.6"
}}
------解决思路----------------------

$a = array(
  array(
    "pla_id" => "298",
    "user_id" => "1",
    "due_in" => "8.33",
  ),
  array(
    "pla_id" => "976",
    "user_id" => "1",
    "due_in" => "338.90",
  ),
  array(
    "pla_id" => "56",
    "user_id" => "2",
    "due_in" => "25.6",
  )
);
$b = array();
foreach($a as $v) {
  if(! isset($b[$v['user_id']])) $b[$v['user_id']] = $v;
  else $b[$v['user_id']]["due_in"] .= ',' . $v["due_in"];
}
var_dump(array_values($b));

array(2) {
  [0]=>  array(3) {
    ["pla_id"]=>    string(3) "298"
    ["user_id"]=>    string(1) "1"
    ["due_in"]=>    string(11) "8.33,338.90"
  }
  [1]=>  array(3) {
    ["pla_id"]=>    string(2) "56"
    ["user_id"]=>    string(1) "2"
    ["due_in"]=>    string(4) "25.6"
  }
}