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PHP Define Function

In PHP, you can create your own functions. A function is a block of statements that can be used repeatedly in a program. A function will not execute immediately when a page loads, but it will execute by a call to the function.

Here's the basic syntax for defining a function in PHP:

function functionName() {
  // Code to be executed
}

function: It is a keyword in PHP which is used to declare a function.
functionName: It represents the name of the function. The function name should start with a letter or underscore (not a number).

Here's a simple example of a PHP function:

function sayHello() {
  echo "Hello, world!";
}

sayHello();  // Call the function

In this example, we have a function called sayHello() that outputs "Hello, world!". We then call the function using its name followed by parentheses.

Functions with parameters:

You can also define functions with parameters. Parameters are like variables that are used to pass data into functions. Here's an example:

function sayHelloTo($name) {
  echo "Hello, " . $name;
}

sayHelloTo("Alice");  // Outputs: Hello, Alice

In this example, the sayHelloTo() function takes one parameter, $name. When we call the function, we pass the string "Alice" as an argument, which is used as the value of the $name parameter inside the function.

Return values:

Functions can also return values, which can be used as the value of expressions. You can specify a return value using the return statement. Here's an example:

function add($x, $y) {
  return $x + $y;
}

echo add(5, 3);  // Outputs: 8

In this example, the add() function takes two parameters and returns their sum. When we call the function with the arguments 5 and 3, it returns 8, which is then outputted by the echo statement.

How to Create Functions in PHP:

Create a basic function in PHP.

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

greet();

Defining Custom Functions in PHP:

Define custom functions for specific tasks.

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

$result = addNumbers(5, 7);
echo "Sum: $result";

PHP Function Parameters and Arguments:
Utilize parameters and arguments in functions.
```
<?php
function greetUser($name) {
    echo "Hello, $name!";
}

greetUser("John");
```

Returning Values from Functions in PHP:

Return values from functions.

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

$result = square(4);
echo "Square: $result";

PHP Function Naming Conventions:
Follow naming conventions for functions (camelCase).
```
<?php
function calculateSum($a, $b) {
    return $a + $b;
}
```

Using Global Variables in PHP Functions:

Access global variables within functions.

<?php
$globalVar = 42;

function printGlobalVar() {
    global $globalVar;
    echo "Global variable value: $globalVar";
}

printGlobalVar();

PHP Anonymous Functions and Closures:

Define anonymous functions and use closures.

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

$result = $add(3, 5);
echo "Sum: $result";

Recursive Functions in PHP:

Implement recursive functions.

<?php
function factorial($n) {
    if ($n <= 1) {
        return 1;
    } else {
        return $n * factorial($n - 1);
    }
}

$result = factorial(5);
echo "Factorial: $result";

PHP Function Scope and Visibility:

Understand function scope and visibility.

<?php
$globalVar = 42;

function printGlobalVar() {
    echo "Global variable value: $globalVar"; // Error: $globalVar is not in scope
}

printGlobalVar();

PHP Function Overloading and Default Values:

Achieve function overloading using default values.

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

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

Variable-Length Argument Lists in PHP Functions:

Use variable-length argument lists in functions.

<?php
function sum(...$numbers) {
    return array_sum($numbers);
}

$result = sum(1, 2, 3, 4, 5);
echo "Sum: $result";

Declaring Functions in PHP Namespaces:

Declare functions within namespaces.

<?php
namespace MyNamespace;

function myFunction() {
    echo "Function in MyNamespace";
}

myFunction();