Functions in PHP are blocks of code designed to perform specific tasks. They help organize and reuse code, making it easier to manage and maintain. Functions can take inputs, process them, and return outputs. Here’s a detailed overview of how functions work in PHP:

Basic Syntax

Defining a Function:

function functionName($parameter1, $parameter2) {
    // Code to be executed
    return $result;
}

• function: Keyword used to declare a function.
• functionName: The name of the function. Should be unique within the same scope.
• $parameter1, $parameter2: Optional parameters that the function can accept. Parameters are used to pass values into the function.
• return $result;: Optional return statement that outputs a value from the function.

Example:

<?php
function greet($name) {
    return "Hello, " . $name . "!";
}

echo greet("Alice");  // Outputs: Hello, Alice!
?>

Explanation:

• The greet function takes one parameter, $name, and returns a greeting message.
• The echo statement calls the greet function with the argument "Alice" and displays the returned message.

Function Parameters

Functions can have zero or more parameters. Parameters are values passed into the function to be used within its code.

Example with Multiple Parameters:

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

echo add(5, 10);  // Outputs: 15
?>

Explanation:

• The add function takes two parameters, $a and $b, and returns their sum.

Default Values: You can provide default values for parameters. These values are used if the function is called without specifying those arguments.

Example:

<?php
function greet($name = "Guest") {
    return "Hello, " . $name . "!";
}

echo greet();        // Outputs: Hello, Guest!
echo greet("Bob");   // Outputs: Hello, Bob!
?>

Explanation:

• The greet function has a default parameter $name set to "Guest". If no argument is passed, "Guest" is used.

Return Values

Functions can return values using the return keyword. The function execution stops when return is executed, and the specified value is returned to the caller.

Example:

<?php
function square($number) {
    return $number * $number;
}

$result = square(4);
echo $result;  // Outputs: 16
?>

Explanation:

• The square function returns the square of the input number. The returned value is stored in $result and then output.

Variable Scope

Local Scope: Variables defined inside a function are local to that function and cannot be accessed outside it.

Example:

<?php
function myFunction() {
    $localVar = "I am local";
    echo $localVar;
}

myFunction();  // Outputs: I am local
// echo $localVar; // Error: Undefined variable
?>

Global Scope: To access a global variable inside a function, use the global keyword or the $GLOBALS array.

Example with global Keyword:

<?php
$globalVar = "I am global";

function myFunction() {
    global $globalVar;
    echo $globalVar;
}

myFunction();  // Outputs: I am global
?>

Example with $GLOBALS Array:

<?php
$globalVar = "I am global";

function myFunction() {
    echo $GLOBALS['globalVar'];
}

myFunction();  // Outputs: I am global
?>

Function Overloading

PHP does not support function overloading (defining multiple functions with the same name but different parameters). Instead, PHP uses function names that are unique within the same scope.

Anonymous Functions

PHP supports anonymous functions (also known as closures), which are functions without a name. They are often used as callback functions.

Example:

<?php
$square = function($number) {
    return $number * $number;
};

echo $square(5);  // Outputs: 25
?>

Explanation:

• An anonymous function is assigned to the variable $square. It can be called using $square().