A pure function in PHP is a function that always returns the same output for the same input and has no side effects. 1. It must produce consistent output based on input alone. 2. It must not modify or depend on external state such as global variables, files, or databases. 3. It should have no hidden dependencies or observable side effects. Pure functions improve predictability, testability, and maintainability of code and are ideal for utility functions, calculations, and transformations. Examples include formatting currency, converting strings to slugs, and array operations like mapping or filtering. Impure functions may access global variables, perform I/O operations, use random or time functions, or trigger events, all of which introduce unpredictability and complicate testing and debugging.

A pure function in PHP is a function that, given the same input, will always return the same output and has no side effects. That means it doesn’t modify any external state — like global variables, files, or databases — and doesn’t rely on or change anything outside its own scope.

Why purity matters

Pure functions are important because they make your code more predictable, easier to test, and simpler to debug. Since they don’t depend on or alter external data, you can reason about them independently of the rest of your application.

For example:

function add(int $a, int $b): int {
    return $a   $b;
}

This add function is pure. It only uses its inputs and returns a consistent result every time.

On the other hand:

$factor = 2;

function multiplyWithGlobal(int $a): int {
    global $factor;
    return $a * $factor;
}

This one isn't pure because it relies on a global variable, which could change elsewhere in your code and affect the function's behavior.

Characteristics of a pure function

• Consistent output: Same input → same output
• No side effects: Doesn’t modify external state (globals, I/O, etc.)
• No hidden dependencies: Only uses what’s passed into it

These traits make pure functions ideal for reusable logic and functional programming patterns.

When to use pure functions

You don’t have to make every function pure, but using them where appropriate can improve your code quality. They’re especially useful when:

• Writing utility/helper functions
• Doing calculations or transformations
• Creating functions used in testing or caching

For instance, formatting a date based on input or converting an array to JSON are good candidates for pure functions.

Some real-world examples:

• A function that formats currency
• One that converts strings to slugs
• Any mapping or filtering operation on arrays

Side effects to watch out for

Even if a function looks simple, it might still be impure. Common red flags include:

• Accessing or modifying global variables (global, $GLOBALS)
• Reading from or writing to a file/database/session
• Using rand() or time() (because they produce different results)
• Triggering events or logging inside the function

Avoiding these keeps your function clean and predictable.

That’s basically it. Pure functions aren’t a strict rule in PHP, but recognizing them — and knowing when to use them — helps write better, more maintainable code.

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