php - check if class name stored in a string is implementing an interface

I understand that my question is somehow wrong, but I'm still trying to solve this problem.

I have an interface Programmer:

interface Programmer {
    public function writeCode();
}

and a couple of namespaced classes:

Students\BjarneProgrammer (implements Programmer)
Students\CharlieActor (implements Actor)

I have this class names stored in array $students = array("BjarneProgrammer", "CharlieActor");

I want to write a function, that will return an instance of class if it's implementing Programmer interface.

Examples:

getStudentObject($students[0]); - It should return an instance of BjarneProgrammer because it's implementing Programmer.

getStudentObject($students[1]); - It should return false because Charlie is not a Programmer.

I tried it using instanceof operator, but the main problem is that I do not want to instantiate an object if it's not implementing Programmer.

I checked How to load php code dynamically and check if classes implement interface, but there is no appropriate answer as I don't want to create object unless it's returned by function.

Solution

You can use class_implements (requires PHP 5.1.0)

interface MyInterface { }
class MyClass implements MyInterface { }

$interfaces = class_implements('MyClass');
if($interfaces && in_array('MyInterface', $interfaces)) {
    // Class MyClass implements interface MyInterface
}

You can pass the class name as a string as function's argument. Also, you may use Reflection

$class = new ReflectionClass('MyClass');
if ( $class->implementsInterface('MyInterface') ) {
    // Class MyClass implements interface MyInterface
}

Update : (You may try something like this)

interface Programmer {
    public function writeCode();
}

interface Actor {
    // ...
}

class BjarneProgrammer implements Programmer {
    public function writeCode()
    {
        echo 'Implemented writeCode method from Programmer Interface!';
    }
}

Function that checks and returns instanse/false

function getStudentObject($cls)
{
    $class = new ReflectionClass($cls);
    if ( $class->implementsInterface('Programmer') ) {
        return new $cls;
    }
    return false;
}

Get an instance or false

$students = array("BjarneProgrammer", "CharlieActor");
$c = getStudentObject($students[0]);
if($c) {
    $c->writeCode();
}

Update: Since PHP - 5.3.9+ you may use is_a, example taken from PHP Manual:

interface Test {
    public function a();
}

class TestImplementor implements Test {
    public function a() {
        print "A";
    }
}

$testImpl = new TestImplementor();

var_dump(is_a($testImpl, 'Test')); // true

Working example here.