How to remove empty subarrays in a variable-depth multi-dimensional array?

Recursion is necessary as your bottom level subarrays have variable depth.

Code: (Demo)

function removeEmptySubs($array){
    foreach ($array as &$a){  // make $a modifiable by reference
        if(!sizeof($a['subs'])){  // check if 'subs' array is empty
            unset($a['subs']);    // remove it
        }else{
            $a['subs']=removeEmptySubs($a['subs']); // recurse
        }
    }
    return $array;
}

$array=[
    3 => [
            'sponsorid' => 2,
            'user_id' => 3,
            'subs' => [
                    4 => [
                            'sponsorid' => 3,
                            'user_id' => 4,
                            'subs' => [
                                    5 => [
                                            'sponsorid' => 4,
                                            'user_id' => 5,
                                            'subs' => [
                                                    6 => [
                                                            'sponsorid' => 5,
                                                            'user_id' => 6,
                                                            'subs' => [
                                                                ]
                                                        ],
                                                    9 => [
                                                            'sponsorid' => 5,
                                                            'user_id' => 9,
                                                            'subs' => [
                                                                ]
                                                        ],
                                                    10 => [
                                                            'sponsorid' => 5,
                                                            'user_id' => 10,
                                                            'subs' => [
                                                                ]
                                                        ]
                                                ]
                                        ],
                                    12 => [
                                            'sponsorid' => 4,
                                            'user_id' => 12,
                                            'subs' => [
                                                ]
                                        ]
                                ]
                        ],
                    11 => [
                            'sponsorid' => 3,
                            'user_id' => 11,
                            'subs' => [
                                ]
                        ]
                ]
        ]
];
var_export(removeEmptySubs($array));

Output:

array (
  3 => 
  array (
    'sponsorid' => 2,
    'user_id' => 3,
    'subs' => 
    array (
      4 => 
      array (
        'sponsorid' => 3,
        'user_id' => 4,
        'subs' => 
        array (
          5 => 
          array (
            'sponsorid' => 4,
            'user_id' => 5,
            'subs' => 
            array (
              6 => 
              array (
                'sponsorid' => 5,
                'user_id' => 6,
              ),
              9 => 
              array (
                'sponsorid' => 5,
                'user_id' => 9,
              ),
              10 => 
              array (
                'sponsorid' => 5,
                'user_id' => 10,
              ),
            ),
          ),
          12 => 
          array (
            'sponsorid' => 4,
            'user_id' => 12,
          ),
        ),
      ),
      11 => 
      array (
        'sponsorid' => 3,
        'user_id' => 11,
      ),
    ),
  ),
)