PHP Recursively Convert Object to Array

I am trying to recursively convert a php object to an array. The function I wrote is this:

public function object_to_array($obj) {
    $array = (array) $obj;
    foreach ($array as $attribute) {
      if (is_array($attribute)) $attribute = $this->object_to_array($attribute);
      if (!is_string($attribute)) $attribute = (array) $attribute;
    }
    return $array;
}

However, I still end up with objects in my outer array. Why is this? Is my function incorrect?

You want to check if it's an object not an array, though you might do both:

if (is_object($attribute) || is_array($attribute)) $attribute = $this->object_to_array($attribute);
//I don't see a need for this
//if (!is_string($attribute)) $attribute = (array) $attribute;

And from Aziz Saleh, reference $attribute so you can modify it:

foreach ($array as &$attribute) {