Why I am suddenly getting a "Typed property must not be accessed before initialization" error when introducing properties type hints?

Since PHP 7.4 introduces type-hinting for properties, it is particularly important to provide valid values for all properties, so that all properties have values that match their declared types.

A property that has never been assigned doesn't have a null value, but it is on an undefined state, which will never match any declared type. undefined !== null.

For the code above, if you did:

$f = new Foo(1);
$f->getVal();

You would get:

Fatal error: Uncaught Error: Typed property Foo::$val must not be accessed before initialization

Since $val is neither string nor nullwhen accessing it.

The way to get around this is to assign values to all your properties that match the declared types. You can do this either as default values for the property or during construction, depending on your preference and the type of the property.

For example, for the above one could do:

class Foo {

    private int $id;
    private ?string $val = null; // <-- declaring default null value for the property
    private DateTimeInterface $createdAt;
    private ?DateTimeInterface $updatedAt;

    public function __construct(int $id) {
        // and on the constructor we set the default values for all the other 
        // properties, so now the instance is on a valid state
        $this->id = $id;
        $this->createdAt = new DateTimeImmutable();
        $this->updatedAt = new DateTimeImmutable();
    }

Now all properties would have a valid value and the the instance would be on a valid state.

This can hit particularly often when you are relying on values that come from the DB for entity values. E.g. auto-generated IDs, or creation and/or updated values; which often are left as a DB concern.

For auto-generated IDs, the recommended way forward is to change the type declaration to:

private ?int $id = null

For all the rest, just choose an appropriate value for the property's type.

For nullable typed properties you need to use syntax

private ?string $val = null;

otherwise it throws a fatal error.

Since this concept leads to unnecessary fatal errors, I have created a bug report https://bugs.php.net/bug.php?id=79620 - with no success, but at least I tried...