PHP Cast to my class

Question

why this is not possible:

$user = (User) $u[0];

but this is possible

$bool = (boolean) $res['success'];

I use PHP 7.0.

Answer 1

As I know, in PHP you can only cast to some types:

(int), (integer) - cast to integer
(bool), (boolean) - cast to boolean
(float), (double), (real) - cast to float  (real deprecated in PHP 8.0)
(string) - cast to string
(binary) - cast to binary string (PHP 6)
(array) - cast to array
(object) - cast to object
(unset) - cast to NULL (PHP 5) (depracted in PHP 7.2) (removed in 8.0)

(see Type Casting)

Instead you could use instanceof to check of specific type:

if($yourvar instanceof YourClass) {
    //DO something
} else {
    throw new Exception('Var is not of type YourClass');
}

EDIT

As mentioned by Szabolcs Páll in his answer, it is also possible to declare a return type or parameter type, but in that cases an exception (TypeError) will be throwen, if the type does not match.

function test(): string
{
    return 'test';
}

function test(string $test){
    return "test" . $test;
}

Since PHP 7.2 it is also possible to make the types nullable by adding ? in front of them:

function test(): ?string
{
    return null;
}