readonly properties allow you to create immutable objects, or at the very least immutable properties.

That way you can be sure that a value won't be changed by accident after being initialized, throughout the object's life.

It's a very similar concept to constants (set via const or define), albeit with two important differences:

• constants need to be defined on "compilation" time, whereas readonly properties will be set during runtime, usually during on object instantiation (so multiple instances will be able to hold different values*)
• as opposed to constants, that live in the global scope, and in case of class constants their value is tied to the class and not to the instance.

You could achieve the same with a private property only accessible via a getter. E.g., in "the olden days":

class Foo {

    private DateTimeImmutable $createAt;

    public function __construct() {
        $this->createdAt = new DateTimeImmutable();
    }

    public getCreatedAt(): DateTimeImmutable
    {
        return $this->createdAt;
    }
}

$f = new Foo();
echo $f->getCreatedAt()->format('Y-m-d H:i:s');

The only problem with this is that it requires a lot of boilerplate code.

With PHP 8.1. the same could be achieved by doing:

class Foo
{

    public function __construct(
        public readonly DateTimeImmutable $createdAt = new DateTimeImmutable()
    )
    { }

}

$f = new Foo();
echo $f->createdAt->format('Y-m-d H:i:s')