Question

The following code:

class Type {

}

function foo(Type $t) {

}

foo(null);

failed at run time:

PHP Fatal error: Argument 1 passed to foo() must not be null

Why is it not allowed to pass null just like other languages?

Answer 1

PHP 7.1 or newer (released 2nd December 2016)

You can explicitly declare a variable to be null with this syntax

function foo(?Type $t) {
}

this will result in

$this->foo(new Type()); // ok
$this->foo(null); // ok
$this->foo(); // error

So, if you want an optional argument you can follow the convention Type $t = null whereas if you need to make an argument accept both null and its type, you can follow above example.

You can read more.

---

PHP 7.0 or older

You have to add a default value like

function foo(Type $t = null) {

}

That way, you can pass it a null value.

This is documented in the section in the manual about Type Declarations:

The declaration can be made to accept NULL values if the default value of the parameter is set to NULL.