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Using $GLOBALS creates hidden dependencies, making functions harder to test, fragile, and unreusable; 2. It complicates unit testing by requiring global state manipulation, leading to slow, fragile tests; 3. Global state is unpredictable due to uncontrolled modifications, causing bugs and debugging challenges; 4. Code relying on $GLOBALS is less reusable and harder to refactor. Instead, use dependency injection, configuration objects, or service containers. Avoid $GLOBALS for data sharing—it undermines code quality, testability, and maintainability, and in most cases, it’s unnecessary.

Using PHP’s $GLOBALS superglobal might seem convenient at first—after all, it gives you access to all variables in the global scope from anywhere in your code. But relying on it introduces serious risks and long-term maintenance problems. While it’s a built-in feature and technically valid PHP, treating $GLOBALS as a go-to solution for sharing data across functions or files is a practice best avoided. Here’s why.

1. It Encourages Tight Coupling and Hidden Dependencies

When a function uses $GLOBALS, it silently depends on variables defined outside its scope. This breaks one of the core principles of clean code: explicit dependencies.

For example:

function calculateTax() {
    return $GLOBALS['income'] * 0.2;
}

At a glance, there’s no way to know this function relies on a global variable called income. This makes the function:

• Harder to test (you must manually set $GLOBALS['income'])
• Fragile (if income isn’t set, it fails)
• Unreusable (can’t easily use it with different income values)

Instead, pass dependencies explicitly:

function calculateTax($income) {
    return $income * 0.2;
}

Now the dependency is clear, testable, and flexible.

2. It Makes Testing Nearly Impossible

Unit testing relies on isolation. When your functions pull values from $GLOBALS, you have to manipulate global state before each test, which:

• Makes tests slower and more complex
• Risks test contamination (one test affects another)
• Breaks the principle of test independence

Imagine writing a test for a function that reads $GLOBALS['user_id']. You’d have to set that global value before each run and ensure no other test interferes with it. That’s fragile and error-prone.

With dependency injection, testing becomes straightforward:

function getUserData($userId, $database) {
    return $database->fetchUser($userId);
}

You can pass a mock database and any user ID—no global setup needed.

3. Global State Is Inherently Unpredictable

Global variables can be modified from anywhere. If multiple functions read and write to $GLOBALS['config'], you can’t be sure what value it holds at any given moment. This leads to:

• Heisenbugs (bugs that disappear when you try to reproduce them)
• Race conditions in long-running processes
• Difficulty debugging ("Who changed this value?")

This unpredictability grows worse as your application scales. A small script might survive global state, but a larger system will suffer from it.

4. It Hinders Code Reusability and Refactoring

Code that depends on global state is hard to move. You can’t just copy a function to another project—you must also bring along all the global variables it expects. This defeats modularity.

Additionally, refactoring becomes risky. Renaming or restructuring a global variable means hunting down every place it’s used via $GLOBALS, which static analysis tools often can’t track reliably.

Better Alternatives

Instead of using $GLOBALS, consider these more maintainable patterns:

• Dependency Injection: Pass needed values as function parameters or inject them into classes.
• Configuration Objects: Bundle settings into a config class or array and pass it where needed.
• Singletons (sparingly): If you truly need shared state, a well-defined singleton is more controlled than raw globals.
• Service Containers: In larger applications, use a DI container to manage and provide dependencies.

Bottom Line

$GLOBALS gives the illusion of convenience but at the cost of clarity, testability, and maintainability. While it has legitimate uses (like in very limited debugging scenarios), using it for general data sharing is a code smell.

Avoid relying on global state. Write functions and classes that are self-contained, predictable, and easy to test. Your future self—and anyone else who reads your code—will thank you.

Basically, just don’t use $GLOBALS unless you absolutely have to. And in 99% of cases, you don’t.

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