Functions

A function is a named block of code you can call whenever you need it. Instead of copying the same lines into ten places, you write them once inside a function and call it by name. When the logic changes, you fix it in one spot.

You define a function with the function keyword, a name, and parentheses. Parameters go inside the parentheses and act like local variables that receive the values you pass in. The function runs its body and, at the end, returns a value with the return statement.

Calling a function looks like calling a built-in one: doubleIt(5) runs the body with 5 as the parameter. The return value lands where you called it, so you can echo it, store it in a variable, or pass it into another function.

Function names are case-insensitive in PHP, and a function must be defined before you call it. Parameters can have default values, so callers can skip them. As of PHP 8, parameters can also be typed, like int $price or string $name, and PHP enforces them.

Default values

A parameter with a default is optional at the call site. If the caller does not pass a value, PHP uses the default. Defaults must come after all required parameters in the list.

function greet($name, $greeting = "Hello") {
  return "$greeting, $name!";
}
echo greet("Aya");          // Hello, Aya!
echo greet("Aya", "Hi");    // Hi, Aya!

Typed parameters and returns

PHP 7 and 8 allow type declarations. The colon after the parentheses declares the return type. PHP checks both directions: wrong input or wrong return throws a TypeError instead of failing silently.

function doubleIt(int $number): int {
  return $number * 2;
}
echo doubleIt(21); // 42
echo doubleIt("x"); // TypeError

Syntax

<?php
function add($a, $b) {
  return $a + $b;
}

echo add(3, 4); // 7
?>

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