A PHP function is made of three things:

  • Its code
  • Its documentation (using PHPDoc)
  • Its prototype

In PHP, a function prototype (also called “signature”) is made of the name, arguments and, since PHP 7, its return type.

Arguments can be typed, we call this “type-hint”, and since PHP 7 we can type an argument with scalar types (bool, int, string…), but since early PHP has objects, we can type-hint with a class. Also, arguments can have a default value.

Basic example:

public function __construct($a, $b, $c) { /* */ }

Here, no type, but signature is using 3 arguments.

Full example:

public function loginAction(Request $request, string $username, bool $useReferer = true): Response
{
    // ...
}

Here, we have a scalar type-hint for $username and $useReferer, and object type-hint for $request. We even have a default value for $useReferer which is true.

The return type, here, is an Response object type. This means that loginAction() MUST return an instance of the Response class, else PHP throws an exception at runtime.

Return type is similar than argument type: it accepts classes and scalars, but since PHP 7.1, it also accepts a special return type: void.

The void return type means that the return instruction in the method / function must never have argument. With a void return type, we can never do something like return 1; or return $response, else PHP throws an exception at runtime.
We can only do return; with this type.

But nothing prevents doing such things:

function test(): void {
    echo "ok";
    
    return;
}
$a = test();
var_dump($a);
// Outputs:
// "ok"
// null

See the void RFC for more information.