Global php variable is always null

Question

I have a bug in my application which I do not understand at all. I made a minimal example which reproduces the issue:

<?php
class MyClass {

  const ITEMS_PER_STACK = 30;

  public function test(): int {
    global $ITEMS_PER_STACK;
    return $ITEMS_PER_STACK;
  }
}

$a = new MyClass();
echo($a->test());

Expected behavior is an output of 30 while in reality if throws a null exception because the global variable cannot be accessed. Can someone explain it to me why this happens and how to fix this? Would be much appreciated, thanks.

Class constants aren't global variables. The correct syntax is self::ITEMS_PER_STACK — Barmar
– Barmar, Commented Apr 28, 2022 at 22:36

artlung · Accepted Answer · 2022-04-28 22:56:32Z

3

If ITEMS_PER_STACK is declared the way you indicate, the way to refer to it from inside the class is with self::ITEMS_PER_STACK.

So:

<?php
class MyClass {

    const ITEMS_PER_STACK = 30;

    public function test(): int {
        return self::ITEMS_PER_STACK;
    }
}

$a = new MyClass();
echo($a->test());

See: Class Constants

You would use the global keyword to force usage of a variable declared somewhere outside the class, like this:

<?php

$outside_variable = 999;

class MyClass {

    const ITEMS_PER_STACK = 30;

    public function test(): int {
        return self::ITEMS_PER_STACK;
    }

    public function testGlobal(): int {
        global $outside_variable;
        return $outside_variable;
    }
}

$a = new MyClass();
echo($a->testGlobal());

See: Global Scope in PHP

edited Apr 28, 2022 at 22:56

answered Apr 28, 2022 at 22:40

artlung

33.5k19 gold badges76 silver badges124 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Felix Mittermeier Over a year ago

Did not know yet there is a difference. So thanks a lot for the quick and great answer :-) Upvoted

ikyuchukov · Accepted Answer · 2022-04-29 12:03:04Z

While the above answer does tell you the solution, it is not the proper way to do this.

Using globals is highly discouraged and leads to numerous bugs and oddities in your applications.

Your initial approach to use a constant is correct, however you need to call it by referencing the class in which it resides if it's not in the same class (where you use self::ITEMS_PER_STACK) as per the above answer.

If you want to use this constant in another class simply prefix it the name of the class in which it resides.

<?php
class MyClass {

    const ITEMS_PER_STACK = 30;

}

echo MyClass::ITEMS_PER_STACK;

Also note that from PHP 7.1 you can add the visibility of the constant such as:

public const ITEMS_PER_STACK = 30;

See Example #4 https://www.php.net/manual/en/language.oop5.constants.php

Collectives™ on Stack Overflow

Global php variable is always null

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related