cross-posted from: https://chrastecky.dev/post/24

Changing a Readonly Property

So, you know how readonly properties are, well… read-only? Turns out they’re not!

I stumbled upon this mechanism just as PHP started deprecating it — but hey, if you ignore the deprecation warnings, you can still use it up until PHP 9!

A bit of theory first: readonly properties can only be assigned inside a class constructor. After that, they’re supposed to be immutable.

final readonly class ReadonlyClass
{
    public string $someProp;

    public function __construct()
    {
        $this->someProp = 'unchangeable!';
    }
}

The only official way to set such a property outside the constructor is via reflection — but even then, only if the property hasn’t been initialized yet:

final readonly class ReadonlyClass
{
    public string $someProp;
}
$test = new ReadonlyClass();
$reflection = new ReflectionClass(ReadonlyClass::class)->getProperty('someProp');
$reflection->setValue($test, 'changed once!');
var_dump($test->someProp);
$reflection->setValue($test, 'changed twice?');

This produces the predictable result:

string(13) "changed once!"

Fatal error: Uncaught Error: Cannot modify readonly property ReadonlyClass::$someProp

Changing It Multiple Times

Enough stalling — let’s dive in! The magical object that can modify a readonly property (and much more) is ArrayObject.

Normally, you’d use ArrayObject to wrap an array. But it also accepts any object as the backing value — and that’s where the fun begins. Once you know how PHP stores properties internally (which is actually pretty simple), chaos follows.

Let’s start with this class:

final readonly class ReadonlyClass
{
    public string $someProp;
    private string $somePrivateProp;
    protected string $someProtectedProp;

    public function __construct()
    {
        $this->someProp = 'unchangeable?';
        $this->somePrivateProp = 'unchangeable?';
        $this->someProtectedProp = 'unchangeable?';
    }

    public function getSomePrivateProp(): string
    {
        return $this->somePrivateProp;
    }

    public function getSomeProtectedProp(): string
    {
        return $this->someProtectedProp;
    }
}

Now we create an instance and wrap it in an ArrayObject:

$instance = new ReadonlyClass();
$arrayObj = new ArrayObject($instance);

And now comes the fun part:

// simply use the property name for public properties
$arrayObj['someProp'] = 'changeable public!';
// use "\0[FQN]\0[Property name]" for private properties
$arrayObj["\0ReadonlyClass\0somePrivateProp"] = 'changeable private!';
// use "\0*\0[Property name]" for protected properties
$arrayObj["\0*\0someProtectedProp"] = 'changeable protected!';

var_dump($instance->someProp, $instance->getSomePrivateProp(), $instance->getSomeProtectedProp());

This prints:

string(18) "changeable public!"
string(19) "changeable private!"
string(21) "changeable protected!"

And just like that, you’ve changed an unchangeable property. You can modify it as many times as you want. So… what other arcane tricks are possible?

Changing an Enum Value

Enums are basically fancy objects that represent a specific named instance — optionally with a value. The key difference from old userland implementations is that PHP guarantees every enum case is a unique instance that’s always equal to itself, no matter where it’s referenced from.

In other words, an enum is really just a class, and ->value or ->name are plain properties.

enum MyEnum: string {
    case A = 'a';
    case B = 'b';
}

$arrayObj = new ArrayObject(MyEnum::A);
$arrayObj['value'] = 'b';
$arrayObj['name'] = 'C';

var_dump(MyEnum::A->value);
var_dump(MyEnum::A->name);

This prints exactly what you’d expect after reading the previous examples:

string(1) "b"
string(1) "C"

Even more amusing: Running var_dump(MyEnum::A); now prints enum(MyEnum::C).

It won’t actually make it equal to another enum case, but if you use the value somewhere and reconstruct it using MyEnum::from(), you’ll get back MyEnum::B.

If you try to serialize and deserialize it, you’ll get an error — because MyEnum::C doesn’t exist:

var_dump(MyEnum::from(MyEnum::A->value));
var_dump(unserialize(serialize(MyEnum::A)));

The first prints enum(MyEnum::B), while the second throws a warning: Undefined constant MyEnum::C.

Breaking Types

ArrayObject is so powerful that even the type system trembles before it. Types? Mere suggestions!

final class TestTypedClass
{
    public string $str = 'test';
    public bool $bool = true;
    public int $int = 42;
}

$instance = new TestTypedClass();
$arrayObj = new ArrayObject($instance);

$arrayObj['str'] = 5;
$arrayObj['bool'] = 'hello';
$arrayObj['int'] = new stdClass();

var_dump($instance->str, $instance->bool, $instance->int);

Output:

int(5)
string(5) "hello"
object(stdClass)#3 (0) {
}

So if you ever thought “Hmm, this boolean could really use more than two possible values” — now you know how!

Dynamic Properties Everywhere

Some internal classes like Closure, Generator, and DateTime disallow dynamic properties. Nevermore!

$closure = fn () => true;
$arrayObject = new ArrayObject($closure);
$arrayObject['test'] = 'hello';

var_dump($closure->test);
// prints string(5) "hello"

Crashing PHP

And finally — my favourite one! Ever wanted to cause a segmentation fault? Try this:

$exception = new Exception("Hello there!");
$arrayObject = new ArrayObject($exception);
$arrayObject["\0Exception\0trace"] = -1;

var_dump($exception->getTraceAsString());

That gave me one beautiful Segmentation fault (core dumped)!

So, how did you like these all-powerful ArrayObject shenanigans?

They auto-translated Original Poster as Původní plakát where "plakát" does mean "poster" but only the kind that you hang on a wall.

Another win for auto translation.

The time flies really fast! Exactly 2 years ago I was the one and only user of this instance and now there are thousands of us! So happy birthday to Lemmings.world and hope you all enjoy our small corner of the internet!

cross-posted from: https://chrastecky.dev/post/16

Starting with PHP 8.5, you'll be able to do the following:

 public function __construct(
    final public string $someProperty,
) {}

This wasn't possible before, as promoted properties couldn't be declared final.

Perhaps the more interesting part is that you can now omit the visibility modifier if you include final. In that case, the property will default to public:

 public function __construct(
    final string $someProperty, // this property will be public
) {}

Personally, I’m not a fan of this behavior — I prefer explicit over implicit. Fortunately, it can be enforced by third-party tools like code style fixers. Still, I would have preferred if the core required the visibility to be specified.

What do you think? Do you like this change, or would you have preferred a stricter approach?

cross-posted from: https://chrastecky.dev/post/13

This change is quite straightforward, so this won’t be a long article. PHP 8.5 adds support for annotating non-class, compile-time constants with attributes. Compile-time constants are those defined using the const keyword, not the define() function.

Attributes can now include Attribute::TARGET_CONSTANT among their valid targets. Additionally, as the name suggests, Attribute::TARGET_ALL now includes constants as well. The ReflectionConstant class has been updated with a new method, getAttributes(), to support retrieving these annotations.

One particularly useful aspect of this change is that the built-in #[Deprecated] attribute can now be applied to compile-time constants.

As promised, this was a short post, since the change is relatively simple. See you next time—hopefully with a more exciting new feature in PHP 8.5!