PHP Variables, Arrays, and Functions

# CHAPTER 6

PHP Variables, Arrays, and Functions #

1. Introduction #

2. Learning Objectives #

• Create and manipulate Indexed Arrays.

• Build and traverse Associative Arrays (key-value pairs).

• Write custom PHP Functions with parameters and return values.

• Understand variable scope within functions.

3. Beginner-Friendly Explanation #

4. Indexed Arrays #

<?php
// Creating an indexed array of colors
$colors = array("Red", "Green", "Blue");
// Modern short syntax:
$colors = ["Red", "Green", "Blue"];

echo "My favorite color is " . $colors[0]; // Outputs: Red

// Adding to an array
$colors[] = "Yellow"; 

// Counting the items
echo count($colors); // Outputs: 4
?>

5. Associative Arrays (Key-Value Pairs) #

<?php
// Creating an associative array for a User
$user = [
    "username" => "john_doe",
    "email" => "john@example.com",
    "role" => "Admin"
];

// Accessing the data using the Key
echo "User Email: " . $user["email"];

// Looping through an associative array using foreach
foreach ($user as $key => $value) {
    echo $key . " is " . $value . "<br>";
}
?>

6. Functions (Reusable Logic) #

<?php
// Defining a custom function
function calculateTotal($price, $tax_rate) {
    $tax_amount = $price * $tax_rate;
    $total = $price + $tax_amount;
    
    return $total; // Sends the result back to the main code
}

// Calling the function (Using the blender)
$shopping_cart = 100;
$ny_tax = 0.08;

$final_price = calculateTotal($shopping_cart, $ny_tax);
echo "Your total is: $" . $final_price;
?>

7. Variable Scope #

<?php
$discount = 5; // Global variable

function applyDiscount($price) {
    // ERROR: $discount does not exist inside this function!
    // return $price - $discount; 
}
?>

*To fix this, you either pass the discount in as a parameter, or use the global keyword.*

8. Backend Workflow Example: User Verification #

<?php
// Simulated database of banned emails
$banned_emails = ["hacker@bad.com", "spam@bot.com"];

function isUserBanned($email_to_check, $banned_list) {
    // in_array() is a built-in PHP function to search an array
    if (in_array($email_to_check, $banned_list)) {
        return true;
    }
    return false;
}

$new_signup = "spam@bot.com";

if (isUserBanned($new_signup, $banned_emails)) {
    echo "Access Denied. You are banned.";
} else {
    echo "Welcome!";
}
?>

9. Best Practices #

• Single Responsibility Principle: A function should do exactly one thing. If your function is called calculateTax(), it should only calculate math. It should *not* also print HTML to the screen. Return the math, and let a different part of your code handle the printing.

10. Common Mistakes #

• echo vs return in Functions: Beginners often echo the result inside the function. This is bad practice because it forces the function to print text immediately, even if you just wanted to save the result to a variable. Always return the value from the function, and echo it outside the function.

11. Exercises #

1. 1. Create an Associative Array representing a Book (Title, Author, Pages). Write a foreach loop to print all the details to the screen.

12. Coding Challenges #

• Challenge: Write a function called celsiusToFahrenheit($temp). It should take a temperature in Celsius, perform the math ($temp * 9/5) + 32, and return the Fahrenheit value. Call the function with the value 20 and echo the result.

13. MCQs with Answers #

14. Interview Questions #

• Q: Explain the difference between an Indexed Array and an Associative Array. Which one is more commonly used when fetching a single user row from a database?

• Q: What is "Variable Scope" in PHP, and why can't a function automatically read a variable declared at the top of your file?

15. FAQs #

16. Summary #

17. Next Chapter Recommendation #