PHP function is a piece of code that can be reused many times.

Advantage of PHP Functions:

• Code Reusability
• Less Code
• Easy to understand

You can define a function using the function keyword:

1.  Defining a Function:

You can define a function using the function keyword

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

2. Function Parameters:

You can pass parameters to a function:

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

// Call the function with an argument
greetWithName("John");
?>

3. Return Values:

Functions can return values:

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

$result = add(3, 5);
echo "Sum: $result"; // Outputs: Sum: 8
?>

4. Variable Scope:

Variables declared inside a function have local scope and are not accessible outside the function:

<?php
function example() {
    $x = 5; // Local variable
    echo $x;
}

example(); // Outputs: 5
// echo $x; // This would cause an error
?>

5. Global Keyword:

You can use the global keyword to access global variables within a function:

<?php
$globalVar = 10;

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

accessGlobal(); // Outputs: 10
?>

6. Static Keyword:

The static keyword allows a variable to retain its value between function calls:

<?php
function increment() {
    static $counter = 0;
    $counter++;
    echo $counter;
}

increment(); // Outputs: 1
increment(); // Outputs: 2
?>

7. Anonymous Functions (Closures):

You can create functions without a name:

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

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

8. Function Arguments by Reference:

You can pass arguments by reference using the & symbol:

<?php
function addTen(&$num) {
    $num += 10;
}

$value = 5;
addTen($value);
echo $value; // Outputs: 15
?>

9. Recursive Functions:

Functions can call themselves

<?php
function factorial($n) {
    if ($n === 0 || $n === 1) {
        return 1;
    } else {
        return $n * factorial($n - 1);
    }
}

echo factorial(5); // Outputs: 120
?>

These are fundamental concepts related to PHP functions.