php-PHP数组中如果有一个元素相同，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"
}}