Why global state is dangerous in PHP - Challenge Your Understanding

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP code with global state?

Consider this PHP code that uses a global variable. What will it print?

PHP

<?php
$counter = 0;
function increment() {
    global $counter;
    $counter++;
}
increment();
increment();
echo $counter;
?>

A 2

B 0

C 1

D Error: Undefined variable

💡 Hint

Remember that global keyword allows access to the variable outside the function.

The global keyword inside increment() lets the function access and modify the $counter variable declared outside. Each call increases it by 1, so after two calls, it becomes 2.

🧠 Conceptual

intermediate

Why can global state cause bugs in PHP?

Which of the following is the main reason global state is dangerous in PHP?

A Global variables are slower to access than local variables

B Global variables can be changed anywhere, making code unpredictable

C PHP does not support global variables

D Global variables automatically reset after each function call

💡 Hint

Think about how many parts of a program can change a global variable.

Global variables can be modified from any part of the program, which makes it hard to track changes and can cause unexpected bugs.

🔧 Debug

advanced

What error does this PHP code cause due to global state misuse?

Look at this PHP code snippet. What error will it produce?

PHP

<?php
function addOne() {
    $count++;
}
addOne();
echo $count;
?>

A Parse error: syntax error

B Fatal error: Cannot access global variable

C Notice: Undefined variable $count

D Output: 1

💡 Hint

Is $count defined inside the function?

The variable $count is not declared or marked global inside the function, so PHP treats it as undefined, causing a notice.

🚀 Application

advanced

How to avoid global state issues in PHP?

Which approach helps avoid problems caused by global state in PHP?

A Use function parameters and return values instead of global variables

B Declare all variables as global to keep them consistent

C Use global variables only inside classes

D Avoid using functions to keep variables local

💡 Hint

Think about how to keep data flow clear and controlled.

Passing data through function parameters and returning results keeps variables local and predictable, reducing bugs from global state.

❓ Predict Output

expert

What is the output of this PHP code demonstrating global state side effects?

Analyze this PHP code. What will it output?

PHP

<?php
$val = 5;
function multiply() {
    global $val;
    $val *= 2;
}
multiply();
multiply();
echo $val;
?>

A 10

B Error: Undefined variable

C 5

D 20

💡 Hint

Each call doubles the value of $val.

The global variable $val starts at 5. Each call to multiply() doubles it: 5 → 10 → 20.