Readonly constructor parameter without visibility is promoted to public class property.

[bartvanraaij]

Description

https://3v4l.org/AapHF#v8.2.10
The following code:

<?php
class MyClass {
    public function __construct(
         readonly string $var,
    ) {}
}

$a = new MyClass('foo');
var_dump($a->var);

Resulted in this output:

object(MyClass)#1 (1) {
  ["var"]=>
  string(3) "foo"
}
string(3) "foo"

The constructor parameter $var is promoted to a public class property, because it has the readonly keyword.

The Constructor Property Promotion RFC however states that:

When a method parameter is prefixed with one of the visibility keywords public, protected or private, it is considered to be “promoted”.

So without a visibility keyword I would not expect the parameter to be promoted, and be scoped to the constructor method only. I expect the output to look like this, as it would without the readonly keyword:

object(MyClass)#1 (0) {
}

Warning: Undefined property: MyClass::$var in /tmp/preview on line 10
NULL

The source of this bug can be found here, I believe:

php-src/Zend/zend_compile.c

Line 7181 in 3f38105

uint32_t flags = param_ast->attr & (ZEND_ACC_PPP_MASK | ZEND_ACC_READONLY);

You can see here that the parameter is promoted if it has a ZEND_ACC_PPP_MASK keyword or a ZEND_ACC_READONLY.
I think the latter is erroneous, but I'm not a C programmer nor knowledgeable about the PHP source so I could be mistaken.

PHP Version

PHP 8.2.10

Operating System

No response

[iluuu1994]

HI @bartvanraaij. Readonly properties were added after constructor property promotion.

https://wiki.php.net/rfc/readonly_properties_v2

It actually also doesn't explicitly mention that the visibility modifier may be omitted. However, since readonly on parameters is meaningless, it is inferred to target the property.

[Girgias]

This is standard and expected behaviour.

CPP is syntactic sugar which translate:

class MyClass {
    public function __construct(
         readonly string $var,
    ) {}
}

to:

class MyClass {
    readonly string $var
    public function __construct(
         string $var,
    ) {
        $this->var = $var;
    }
}

Which is valid syntax, as a typed property only requires a single modifier, be that a visibility modifier, static, or the most recent one, readonly.

Indeed:

class Foo {
    static string $v;
}

Also compiles into a public static typed property just fine.

[bartvanraaij]

Hi @iluuu1994 and @Girgias , thanks for the quick responses!
Didn't know this was expected behaviour, but after reading your rationale, I guess it makes sense. Thanks.