2017-07-31 - Why casting to Interface in PHP would be useless

example.php

<?php

/**
 * Why casting to Interface in PHP would be useless.
 *
 * In applications I write, all function parameters are type hinted to
 * interfaces except for base types and value objects.  This allows the unit 
 * tests to supply dummy objects.
 *
 * The interfaces themselves have no logic inside, so the only test that they
 * need to pass, is that they have the required functions.  Unfortunately,
 * inside PHP, there really is no simple way to do this.
 *
 * Therefore, I decided to test the interface using the first class that 
 * implements it.  Therefore, when I did the 'get to red' step of TDD and 
 * 'Write only enough code to get the test to fail' I could write 
 * `$Object->NewMethod();` and the test would fail.  In order to 'get to green' 
 * I would be forced to add the function BOTH to the interface and to it's 
 * implementing class.
 *
 * Unfortunately, as you can see below, that doesn't work in PHP.  Basically,
 * it is called type 'hinting' for a reason.
 *
 * @author    Matthew "Juniper" Barlett <emeraldinspirations@gmail.com>
 * @copyright 2017 Matthew "Juniper" Barlett <emeraldinspirations@gmail.com>
 * @license   MIT
 */

interface DemoInterface
{

    function FunctionInBothClassAndInterface();

}

class DemoClass implements DemoInterface
{

    public function FunctionInBothClassAndInterface() {
        return 'I work as I should!';
    }

    public function FunctionNotInInterface() {
        return 'Developer forgot to add to interface, not exposed in unit test';
    }

}

function UnitTest()
{

    $Variable = (DemoInterface) new DemoClass();
    // PHP Parse error:  syntax error, unexpected 'new' (T_NEW) in ...
    
    $Variable = (DemoInterface) (new DemoClass());
    // PHP Notice:  Use of undefined constant DemoInterface - assumed ...
    
    $TypeCastToInterface = function (DemoInterface $Object) : DemoInterface {
        return $Object;
    };

    $Variable = $TypeCastToInterface(new DemoClass());

    var_dump($Variable);
    /*
    object(DemoClass)#2 (0) {
    }
    */
    
    $Variable->FunctionNotInInterface();
    // Unit test SHOULD fail, but PHP does not detect issue

}

UnitTest();