The two main tools for accessing global variables in PHP are the global keyword and the $GLOBALS superglobal array; 1) The global keyword creates a reference to a global variable inside a function, allowing direct access and modification, and if the variable is undefined, it initializes it as null; 2) The $GLOBALS array provides direct access to all global variables via array syntax, does not require declaration, and accessing an undefined key triggers a notice; 3) global is preferred for simplicity and readability when working with known variables, while $GLOBALS is better for dynamic or conditional access; 4) Both introduce side effects by enabling global state modification, so they should be used sparingly to maintain code testability and avoid bugs; 5) Performance-wise, global has a slight edge due to direct referencing, whereas $GLOBALS involves array lookup overhead—choosing between them depends on context, clarity, and need for dynamic access.

When working with PHP, managing variable scope—especially access to global variables from within functions—can be a common source of confusion. Two tools stand out for this purpose: the global keyword and the $GLOBALS superglobal array. While they both allow access to global variables inside functions, they work differently under the hood and have distinct use cases. Let’s break down the differences and clear up the confusion.

What the global Keyword Does

The global keyword tells PHP that you want to use a variable from the global scope inside a function. It essentially creates a reference to the global variable, making it available locally.

$color = 'blue';

function showColor() {
    global $color;
    echo $color;
}

showColor(); // Outputs: blue

Here, global $color; links the local $color inside the function to the globally defined $color. Any changes made to $color inside the function will affect the global variable.

You can also declare multiple globals at once:

global $var1, $var2, $var3;

Key Point: global creates a reference. It doesn’t copy the value—it links to the original variable.

How $GLOBALS Works

$GLOBALS is a superglobal array that holds all global variables as key-value pairs, where the key is the variable name (as a string) and the value is the current value of that variable.

$color = 'blue';

function showColor() {
    echo $GLOBALS['color'];
}

showColor(); // Outputs: blue

Unlike global, $GLOBALS is always available and doesn’t require a declaration. You access global variables using array syntax.

You can also modify global variables directly:

function changeColor() {
    $GLOBALS['color'] = 'red';
}

After calling changeColor(), the global $color becomes 'red'.

Key Differences Between global and $GLOBALS

Practical Considerations

• Use global for simplicity and readability when you need to work with a few global variables in a function. It's cleaner and more straightforward.

• Use $GLOBALS when you need dynamic access, such as looping through global variables or conditionally accessing them by name.

  foreach (['color', 'size', 'shape'] as $var) {
      if (isset($GLOBALS[$var])) {
          echo $GLOBALS[$var];
      }
  }

• Avoid overusing either—heavy reliance on global state makes code harder to test and maintain. Consider dependency injection or configuration objects instead.

• Be cautious with side effects: Both methods let you modify global state, which can lead to bugs if not carefully managed.

A Subtle Gotcha

There’s a subtle behavior when using global with uninitialized variables:

function test() {
    global $undefined;
    var_dump($undefined); // NULL, even if not previously defined
}

PHP initializes $undefined as null in the global scope when you declare it with global. This doesn’t happen with $GLOBALS['undefined'], which will trigger a notice if accessed without prior definition.

In short:
global is like saying, “I want to work with this global variable directly.”
$GLOBALS is like saying, “Give me a map of all global variables—I’ll look up the one I need.”

Both get the job done, but choosing the right one depends on clarity, context, and whether you need dynamic access.

Basically, know your tools—and use them wisely.

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