🔹 1. What Is a Function?

A function is a block of code that performs a specific task and can be reused anywhere in your script.
It allows you to write once, use many times — saving time and avoiding repetition.

<?php
function greetUser() {
    echo "Welcome to StreetGeek Academy!";
}

greetUser(); // Call the function
?>

✅ Output:
Welcome to StreetGeek Academy!

🔹 2. Function Syntax

function functionName(parameters) {
    // Code to be executed
    return value; // (optional)
}

• Functions must start with the keyword function
• Names cannot start with a number
• Parentheses are required even if no parameters are passed
• return is optional but useful to send a value back

🧩 Example with Parameters

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

greet("Yogi"); // Output: Hello, Yogi!

greet("Brandon");
greet("Nicole");
?>

🔹 3. Functions That Return Values

Instead of echoing directly, functions can return values for flexible reuse.

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

$result = add(10, 5);
echo "Sum: $result";
?>

✅ Output:
Sum: 15

🧠 Tip: Always use return when you need to pass a value back to the caller.

🔹 4. Default Parameter Values

If a parameter isn’t provided, PHP can use a default value.

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

greet();          // Hello, StreetGeek!
greet("Yogi");    // Hello, Yogi!
?>

🔹 5. Variable Scope

The scope of a variable defines where it can be accessed.

🧩 Example: Local vs Global

<?php
$city = "Richmond";

function showCity() {
    global $city; // Access global variable
    echo "We’re learning PHP in $city!";
}

showCity();
?>

✅ Output:
We’re learning PHP in Richmond!

🧩 Example: Static Variable

<?php
function counter() {
    static $count = 0;
    $count++;
    echo "Count: $count<br>";
}

counter();
counter();
counter();
?>

✅ Output:
Count: 1
Count: 2
Count: 3

🧠 Without static, $count would reset to 0 each time.

🔹 6. Built-in String and Array Functions

PHP comes with hundreds of built-in functions. Here are a few essentials:

🔸 String Functions

🔸 Array Functions

🧩 Hands-On Practice

Exercise 1: Simple Greeting Function

Create greet.php:

<?php
function greet($name) {
    echo "Hello, $name! Welcome to StreetGeek Academy.<br>";
}

greet("Yogi");
greet("Nicole");
greet("Brandon");
?>

✅ Output: Each name displays a personalized greeting.

Exercise 2: Addition Function

Create add.php:

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

echo "10 + 5 = " . addNumbers(10, 5);
?>

✅ Output:
10 + 5 = 15

Exercise 3: Default Parameter

Create welcome.php:

<?php
function welcome($name = "StreetGeek Student") {
    echo "Welcome back, $name!<br>";
}

welcome();          // Uses default
welcome("Yogi");    // Custom name
?>

Exercise 4: Global and Local Scope

Create scope.php:

<?php
$academy = "StreetGeek";

function showAcademy() {
    global $academy;
    echo "You’re learning at $academy Academy.";
}

showAcademy();
?>

✅ Output:
You’re learning at StreetGeek Academy.

Exercise 5: Static Counter

Create count.php:

<?php
function increment() {
    static $count = 0;
    $count++;
    echo "This function has been called $count times.<br>";
}

increment();
increment();
increment();
?>

✅ Output increments each call.

🎯 Mini Project – BMI Calculator

Goal: Build a function that calculates Body Mass Index and returns a health category.

Create bmi.php:

<?php
function calculateBMI($weight, $height) {
    $bmi = $weight / ($height * $height);
    return $bmi;
}

function getBMICategory($bmi) {
    if ($bmi < 18.5) return "Underweight";
    elseif ($bmi < 25) return "Normal";
    elseif ($bmi < 30) return "Overweight";
    else return "Obese";
}

$weight = 70;   // kg
$height = 1.75; // meters

$bmi = calculateBMI($weight, $height);
echo "Your BMI is " . round($bmi, 1) . " - " . getBMICategory($bmi);
?>

✅ Output example:
Your BMI is 22.9 – Normal

🧠 Combines math, conditionals, and multiple functions into one working system.

🧾 Module 6 Quiz

💪 Challenge Task – Dynamic Calculator

Objective: Create a PHP calculator using functions for add, subtract, multiply, and divide.

Create calculator.php and add these functions:

<?php
function add($a, $b) { return $a + $b; }
function subtract($a, $b) { return $a - $b; }
function multiply($a, $b) { return $a * $b; }
function divide($a, $b) {
    return $b == 0 ? "Cannot divide by zero" : $a / $b;
}

$x = 10;
$y = 5;

echo "Add: " . add($x, $y) . "<br>";
echo "Subtract: " . subtract($x, $y) . "<br>";
echo "Multiply: " . multiply($x, $y) . "<br>";
echo "Divide: " . divide($x, $y);
?>

✅ Output:
Add: 15
Subtract: 5
Multiply: 50
Divide: 2

🧾 Submission Checklist

• ✅ greet.php → personalized greetings
• ✅ add.php → returns correct math result
• ✅ bmi.php → functional BMI calculator
• ✅ calculator.php → function-based calculator
• ✅ Quiz answers filled

🏁 Next Step: In Module 7, you’ll take everything you’ve learned and apply it to forms and user
input validation, learning to handle and protect user-submitted data — a crucial step toward secure web
development and building WordPress-like form workflows.