Global keyword behavior in PHP - Time & Space Complexity

Time Complexity: Global keyword behavior

O(1)

Understanding Time Complexity

Let's explore how using the global keyword affects the time it takes for PHP code to run.

We want to see how the program's steps grow when accessing global variables inside functions.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

<?php
$count = 0;
function increment() {
    global $count;
    for ($i = 0; $i < 1000; $i++) {
        $count++;
    }
}
increment();
?>
    

This code uses the global keyword to access a variable outside the function and increments it 1000 times.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The for loop that runs 1000 times.
• How many times: The loop runs exactly 1000 times each time the function is called.

How Execution Grows With Input

Explain the growth pattern intuitively.

Input Size (n)	Approx. Operations
10	10 increments
100	100 increments
1000	1,000 increments

Pattern observation: The number of steps grows directly with the number of times the loop runs, so doubling the loop doubles the work.

Final Time Complexity

Time Complexity: O(1)

This means the time to run is constant because the loop runs a fixed 1000 times regardless of input size.

Common Mistake

[X] Wrong: "Using global makes the code slower because it searches the whole program every time."

[OK] Correct: Accessing a global variable with global is a simple reference and does not add extra loops or searches; the main time cost is still the loop itself.

Interview Connect

Understanding how global variables affect performance helps you write clear and efficient code, a skill valued in many programming tasks.

Self-Check

"What if we replaced the global keyword with passing the variable as a function argument? How would the time complexity change?"