Global php variable is always null

April 29, 2022

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.

>Solution :

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