PHP Functions

What are Functions?

Functions are reusable blocks of code that perform specific tasks. They help organize code, reduce repetition, and make programs more modular. PHP provides both built-in functions and the ability to create custom functions.

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Declaring Functions

Functions are declared using the function keyword:

function greet() {
    echo "Hello, World!";
}

greet();  // Hello, World!

Naming Rules

// Valid function names
function sayHello() { }
function say_hello() { }
function _privateFunction() { }

// Invalid: Cannot start with number or contain hyphens
// function 2sayHello() { }
// function say-hello() { }

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Parameters

Functions can accept parameters to make them more flexible.

Basic Parameters

function greetUser($name) {
    echo "Hello, $name!";
}

greetUser("John");  // Hello, John!

Default Values

function greetUser($name = "Guest", $greeting = "Hello") {
    echo "$greeting, $name!";
}

greetUser();              // Hello, Guest!
greetUser("John", "Hi");  // Hi, John!

Pass by Reference

By default, functions receive a copy of the variable. Use & to modify the original:

function increment(&$number) {
    $number++;
}

$value = 5;
increment($value);
echo $value;  // 6

Variable-Length Arguments

The splat operator (...) collects arguments into an array:

function sumNumbers(...$numbers) {
    return array_sum($numbers);
}

echo sumNumbers(1, 2, 3, 4, 5);  // 15

Named Arguments (PHP 8.0+)

Specify arguments by name to skip optional parameters:

function createUser($name, $email, $age = null, $active = true) {
    return ['name' => $name, 'email' => $email, 'age' => $age, 'active' => $active];
}

$user = createUser(name: "John", email: "john@example.com", active: false);

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Type Hints

Type hints enforce parameter and return types. If you pass a value of the wrong type, PHP throws a TypeError at runtime and the function will not execute.

function createUser(string $name, int $age, bool $active = true): array {
    return ['name' => $name, 'age' => $age, 'active' => $active];
}

createUser("John", 25);        // Works
createUser("John", "twenty");  // TypeError: Argument #2 must be of type int

Nullable Types

Use ? to allow null:

function findUser(?int $id): ?array {
    if ($id === null) {
        return null;
    }
    return ['id' => $id, 'name' => 'John'];
}

Union Types (PHP 8.0+)

Accept multiple types:

function process(int|float $number): int|float {
    return $number * 2;
}

Common Type Hints

Type	Description
string, int, float, bool	Scalar types
array	Array type
object	Any object
callable	Callable (function/closure)
mixed	Any type (PHP 8.0+)
void	No return value

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Return Values

Basic Returns

function multiply($a, $b) {
    return $a * $b;
}

echo multiply(4, 5);  // 20

Returning Arrays

function getUserData($id) {
    return [
        'id' => $id,
        'name' => 'John Doe',
        'email' => 'john@example.com'
    ];
}

$user = getUserData(1);
echo $user['name'];  // John Doe

Early Returns

function validateAge($age) {
    if ($age < 0) {
        return "Age cannot be negative";
    }
    return "Valid age";
}

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Static Variables

Static variables retain their value between function calls:

function countCalls() {
    static $count = 0;
    $count++;
    return $count;
}

echo countCalls();  // 1
echo countCalls();  // 2
echo countCalls();  // 3

For more on variable scope (global, local, superglobals), see Variables.