PHP recursive function returning null array

I have a huge array in which keys are also not constant in most of the cases, but there are 3 keys that always constant (#name,#default_value,#value) and #default_value and #value is different i want to get these kind of sub arrays in 1 simple array , for this purpose i am using recursion in whole array and checking out if these 3 keys are present there i can print these values inside recursion easily but I am not able to get those values in return. So that i can precess them further.

$form=array();
$form['field_one']['#name']="field_one";
$form['field_one']['#default_value']="old";
$form['field_one']['#value']="new";
$form['field_two']['#name']="field_two";
$form['field_two']['#default_value']="old";
$form['field_two']['#value']="new";
$form['field_three']['#name']="field_three";
$form['field_three']['#default_value']="old";
$form['field_three']['#value']="old";
$form['thiscouldbeanotherarray']['idk']['field_four']['#name']="field_four";
$form['thiscouldbeanotherarray']['idk']['field_four']['#default_value']="old";
$form['thiscouldbeanotherarray']['idk']['field_four']['#value']="new";
$arr_get = get_updatedvalues($form,array());
var_dump($arr_get);

function get_updatedvalues($form,$fields) {  
    if (!is_array($form)) {
        return;
    }

    foreach($form as $k => $value) { 
        if(str_replace('#','',$k) =='default_value' && ($form['#default_value'] != $form['#value'] ) ){               
            $fields['field'][$form['#name']] = array("name" => $form['#name'],"old_value" =>$form['#default_value'],"new_value" =>$form['#value']);              
            var_dump($fields);
        }
        get_updatedvalues($value,$fields);     
    }   
    return $fields;
}

If you run this code it will give you $arr_get > array (size=0), there i need all three values

Answer

If I understand correctly, you need to pass fields as a reference in order to change it:

function get_updatedvalues($form, &$fields) {

To do that, however, you’ll need to change your initial call so that you have a variable to pass as the reference:

$array = [];
$arr_get = get_updatedvalues($form,$array);

Running this I get:

Array
(
    [field] => Array
        (
            [field_one] => Array
                (
                    [name] => field_one
                    [old_value] => old
                    [new_value] => new
                )

            [field_two] => Array
                (
                    [name] => field_two
                    [old_value] => old
                    [new_value] => new
                )

            [field_four] => Array
                (
                    [name] => field_four
                    [old_value] => old
                    [new_value] => new
                )

        )

)