Solve the Typed property X must not be accessed before initialization error

To solve this problem, you need to know that PHP initializes the parameters of a class differently depending on whether they are type parameters or not. For example let's take the following legacy code:

class UserA {
    public $id;
    public $userName;
}

$userA = new UserA();

echo '<pre>';
var_dump($userA);
echo '</pre>';

echo 'The $userA->id is: >' . $userA->id . '<';

The output looks like this:

object(UserA)#1 (2) {
  ["id"]=>
  NULL
  ["userName"]=>
  NULL
}

The $userA->id is: ><

As you can see the class properties are automatically initialized with NULL values.

And now use typed properties:

class UserB {
    public ?int $id;
    public string $userName;
}

$userB = new UserB();

echo '<pre>';
var_dump($userB);
echo '</pre>';

echo 'The $userB->id is: >' . $userB->id . '<';

Where the output is:

object(UserB)#1 (0) {
  ["id"]=>
  uninitialized(?int)
  ["userName"]=>
  uninitialized(string)
}

Fatal error: Uncaught Error: Typed property UserB::$id must not be accessed before initialization in ...

As you can see in this case the value of the class propertik has not been initialized. They were simply left uninitialized .

And this brings us to the source of the problem. The id variable is defined to be either int or null . But the uninitialized value is neither int nor null .

The solution is simple. For typed properties, you always have to initialize the variable explicitly. 

You can do this either by specifying a default value:

class UserB {
    public ?int $id = null;
    public string $userName;
}

or by using the constructor:

class UserB {
    public ?int $id;
    public string $userName;

    /**
     * UserB constructor.
     * @param int|null $id
     * @param string $userName
     */
    public function __construct(?int $id, string $userName)
    {
        $this->id = $id;
        $this->userName = $userName;
    }
}

PHP type declarations