Why Do I Get a \"Typed Property Must Not Be Accessed Before Initialization\" Error in PHP?

Understanding "Typed Property Must Not Be Accessed Before Initialization" Error in Properties with Type Hints

PHP 7.4 introduced type-hinting for properties, necessitating proper initialization of all properties. When a property lacks an assigned value, it remains in an undefined state that could conflict with its declared type.

For instance, if we declare:

class Foo {
    private int $id;
    private ?string $val;
    private DateTimeInterface $createdAt;
    private ?DateTimeInterface $updatedAt;
}

Accessing $val without prior initialization would result in the error: "Typed property Foo::$val must not be accessed before initialization." This is because $val is neither a string nor null when accessed.

To resolve this, assign valid values to all properties, either as default values or during construction. For the above example, we could use:

class Foo {
    private int $id;
    private ?string $val = null;
    private Collection $collection;
    private DateTimeInterface $createdAt;
    private ?DateTimeInterface $updatedAt;

    public function __construct(int $id) {
        // Setting default values for other properties
        $this->id = $id;
        $this->createdAt = new DateTimeImmutable();
        $this->updatedAt = new DateTimeImmutable();

        $this->collection = new ArrayCollection();
    }
}

By doing so, all properties have valid values and the instance is in a valid state.

This error can commonly occur when relying on database-generated values such as IDs or timestamps. For auto-generated IDs, it's advisable to change the type declaration to private ?int $id = null. For other values, choose suitable defaults that match the property's type.

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