Why Can't I Directly Call an Object's Closure Property in PHP, and How Can I Fix It?

Calling Object Property Closure Directly

When assigning a closure to an object's property, you may encounter difficulties calling it directly without reassigning the closure to a variable. Consider the following example:

$obj = new stdClass();
$obj->callback = function() {
    print "HelloWorld!";
};

Unfortunately, attempting to call $obj->callback() will result in the error "Fatal error: Call to undefined method stdClass::callback()".

PHP 7 Solution

In PHP 7, using arrow functions (introduced in PHP 7.4), you can directly invoke a closure assigned to an object's property as follows:

$obj = new StdClass;
$obj->fn = function($arg) { return "Hello $arg"; };
echo ($obj->fn)('World');

Closure::call()

For versions of PHP below 7, an alternative is to use the Closure::call() function. However, this method is not applicable to StdClass instances.

Magic __call Method

Before PHP 7, a viable approach was to implement the magic __call method, which allows intercepting calls to undefined methods. In the following example, a Foo class is created with an __call implementation:

class Foo
{
    public function __call($method, $args)
    {
        if(is_callable(array($this, $method))) {
            return call_user_func_array($this->$method, $args);
        }
        // else throw exception
    }
}

$foo = new Foo;
$foo->cb = function($who) { return "Hello $who"; };
echo $foo->cb('World');

Note that the __call implementation avoids infinite recursion by preventing the same method from being called within the __call body.

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