How should I PHPDoc a callback?

Question

I have a method which extracts a modified preorder tree transversal tree from the database, and filters that using a callback function. For example:

/**
 * Recursive function for building the Cas_Template_TreeNode.
 *
 * @static
 * @param array $rows
 * @param callback $filter A function to filter the tree by (return a value convertible to false to remove the item from the tree)
 * @return array
 */
private static function MakeTreeGivenDbRows($rows, $filter = null)
{
    if ($filter === null)
    {
        $filter = function($unused)
        {
            return true;
        };
    }
    $result = array();
    $childrenCount = 0;
    for ($idx = 0; $idx < count($rows); $idx += $childrenCount + 1)
    {
        $current = $rows[$idx];
        $childrenCount = self::ChildrenCountFromRow($current);
        if (!$filter($current))
        {
            continue;
        }
        $childrenStartAt = $idx + 1;
        $childRows = array_slice($rows, $childrenStartAt, $childrenCount);
        $children = self::MakeTreeGivenDbRows($childRows, $filter);
        $result[] = new Cas_Template_TreeNode(self::MakeNodeGivenDbRow($current), $children);
    }
    if (empty($result))
    {
        return null;
    }
    return $result;
}

I'm not sure what the PHPDoc should be for the variable $filter -- it's a callback, which is what I've indicated, but I'm not sure if that's correct.

Also, any other comments on the quality (or lack thereof) in this code would be appreciated :)

Answer 1

The correct type-hint is callable, which has been available in e.g PhpStorm for a long time, and is part of PSR-5 which is currently under specification.

(I'm surprised nobody else mentioned callable, it's nothing new and has been used everywhere for years - to my knowledge, callback is not and never was a defined PHP pseudo-type)

Note that callable includes not just closures, but also "old school" PHP callbacks, e.g. array($object, 'methodName') or array('ClassName', 'methodName') and even 'function_name'- to maximize the usefulness of your API, you should cover all of those use-cases, which is quite easy, since both call_user_func and call_user_func_array support all four varieties of callables: function-name as string, object/method-name, class/method-name and closure.