数组合并，相同键名合并一起

$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));







//结果

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"
  }
}