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Constants are no longer constant in PHP

hbgl on January 31, 2024

I really hope that I am not opening a can of worms here because I think I have found something in PHP that I should not have. Something that should...
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mcharytoniuk profile image
Mateusz Charytoniuk • • Edited

const means a constant reference; you cannot change scalar values without changing a reference to them, but you can keep the same reference to an object and change it's properties. I think it has always been like that, so the title is a bit misleading.

If you want immutable objects, use readonly classes:

readonly class MyClass { ... }
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If you want to use 3rd party class and make sure nobody modifies it, wrap it in a readonly class

readonly class MyWrapper 
{
    public function __construct(private OtherClass $otherClass) {} 

    /* ... */
}
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Other languages use that approach also (JS for example), it's just a matter of how a language defines it. We still have ways to enforce immutability, I don't see an issue with that.

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hbgl profile image
hbgl •

Until PHP 8.1, const really meant "constant data", i.e. deep immutability. The idea of a constant reference did not exist. The keyword const in JS and PHP have always worked differently. In JS, const refers to a variable binding that cannot be rebound to another piece of data. In that sense, const in JS it is more like readonly in PHP.

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mcharytoniuk profile image
Mateusz Charytoniuk •

I can't finy any info on const changes, and that would be a significant think, it's not listen in 8.1 changelog either: php.net/releases/8.1/en.php

I think you might be mistaken. Unless you can point me to a changelog that mentions that?

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hbgl profile image
hbgl •

I believe that const objects were an unintentional side-effect of allowing const enum cases. Therefore it only makes sense that there is no mention in the changelog.

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ravavyr profile image
Ravavyr •

Just use define('BOB','somevalue') like anyone who writes PHP would normally do it.

Hell even PHP's documentation says so:
php.net/manual/en/language.constan...

To sum up, use "define()" for global constants in an application and use "const" for in-class-scope constants.

Personally, i'll just use variables for everything because the only "constant" in web dev is that everything changes...eventually.

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romson profile image
Roman •

I also didn't encounter a situation yet when using constant objects was a suitable option. But I'd like to point out that from the technical perspective, the mutability of the object should not be confusing when it's used with a constant because objects are reference types. Therefore, the constant does not contain an object, rather it contains a reference to the object and that reference is constant but the internal state of the object can be changed. You still can't assign a new value (a reference to another object) to the constant, so the constant can not be changed.

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hbgl profile image
hbgl • • Edited

I believe the distinction between object references and object data is not suitable when we are talking about constants. A constant should refer to deeply immutable data, like π being equal to 3.14. From this perspective, having a constant reference is a bit of a nonsensical concept.

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citronbrick profile image
CitronBrick •

When you say "no longer" do you meant that previously in PHP const objects were not mutable ?

Is this a change or a discovery ?

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hbgl profile image
hbgl •

Before PHP 8.1, you could not have const objects at all. Only scalars and arrays were allowed which are effectively immutable. Objects are and always have been mutable in PHP.