property.readOnlyAssignOutOfClass

Code example

<?php declare(strict_types = 1);

class Foo
{
	public readonly int $bar;

	public function __construct(int $bar)
	{
		$this->bar = $bar;
	}
}

class Bar extends Foo
{
	public function __construct(int $bar)
	{
		parent::__construct(1);
		$this->bar = $bar;
	}
}

Why is it reported?

PHP's readonly properties can only be assigned from within the class that declares them. Assigning a readonly property from a child class, an unrelated class, or a standalone function is not allowed, even if the property's visibility (public or protected) would otherwise permit access.

In the example above, Bar attempts to assign $this->bar inside its constructor, but $bar is declared as readonly in Foo. Since Bar is not the declaring class, this assignment is forbidden.

How to fix it

Pass the value to the parent constructor so the declaring class performs the initialization:

 <?php declare(strict_types = 1);

 class Bar extends Foo
 {
 	public function __construct(int $bar)
 	{
-		parent::__construct(1);
-		$this->bar = $bar;
+		parent::__construct($bar);
 	}
 }

Or move the readonly property to the class that needs to assign it:

 <?php declare(strict_types = 1);

 class Bar extends Foo
 {
+	public readonly int $baz;
+
 	public function __construct(int $bar)
 	{
 		parent::__construct(1);
-		$this->bar = $bar;
+		$this->baz = $bar;
 	}
 }