PHP Basics, Arrays, Functions & OOP

TAKEAWAYS:

• PHP is a server-side scripting language embedded in HTML to process form data, validate inputs, and generate dynamic content.
• Arrays store multiple values in a single variable (indexed, associative, or multidimensional) and are essential for handling lists (e.g., user hobbies, product inventories).
• Functions encapsulate reusable logic (e.g., form validation, calculations) and reduce code duplication.
• OOP in PHP uses classes/objects to model real-world entities (e.g., User, Order) with properties and methods, improving maintainability.
• Cookies and sessions manage persistent data across page requests (e.g., shopping carts, user logins).
• JSON and PHP arrays often interchange for API data (e.g., Daraz’s product search results).

1. Introduction to PHP

PHP (Hypertext Preprocessor) is a server-side scripting language embedded in HTML to create dynamic web pages. Unlike JavaScript (client-side), PHP executes on the server before sending HTML to the browser. Key features:

• Platform-independent: Runs on Linux/Windows with Apache/Nginx.
• Database integration: Works seamlessly with MySQL, PostgreSQL, etc.
• Form processing: Validates and handles user inputs (e.g., login forms).

sequenceDiagram
    participant User
    participant Browser
    participant Server
    participant PHP
    participant Database

    User->>Browser: Sends HTTP request (e.g., form submission)
    Browser->>Server: POSTs data to `process.php`
    Server->>PHP: Executes PHP script
    PHP->>Database: Queries/updates data (e.g., user login)
    PHP->>Server: Returns dynamic HTML
    Server->>Browser: Sends HTML response
    Browser->>User: Displays result (e.g., dashboard)
    note right of Browser: **PHP executes server-side, unlike JavaScript**

How PHP Works

sequenceDiagram
    participant User
    participant Browser
    participant Server
    participant PHP
    participant Database

    User->>Browser: Sends HTTP request (e.g., form submission)
    Browser->>Server: POSTs data to `process.php`
    Server->>PHP: Executes PHP script
    PHP->>Database: Queries/updates data
    PHP->>Server: Returns dynamic HTML
    Server->>Browser: Sends HTML response
    Browser->>User: Displays result

Real Picture

2. PHP Syntax Basics

PHP code is enclosed in <?php ... ?> tags and mixed with HTML.

Variables

• Naming rules: Start with $, case-sensitive ($name ≠ $Name).
• Data types: string, int, float, bool, array, object.
• Example:

  $name = "Rohan";       // String
  $age = 20;             // Integer
  $isStudent = true;     // Boolean
  echo $name . " is " . $age . " years old."; // Output: Rohan is 20 years old.
  

Operators

3. Arrays in PHP

Arrays store multiple values in one variable. Three types:

a) Indexed Arrays

• Numeric keys (auto-incremented from 0).
• Example: List of fruits.

  $fruits = array("Apple", "Banana", "Mango");
  echo $fruits[1]; // Output: Banana
  

b) Associative Arrays

• Named keys (e.g., ["name"] => "Rohan").
• Example: User profile.

  $user = array(
      "name" => "Rohan",
      "email" => "rohan@example.com",
      "age" => 20
  );
  echo $user["email"]; // Output: rohan@example.com
  

c) Multidimensional Arrays

• Arrays within arrays.
• Example: Product catalog (Daraz-style).

  $products = array(
      array("id" => 1, "name" => "Laptop", "price" => 50000),
      array("id" => 2, "name" => "Phone", "price" => 20000)
  );
  echo $products[0]["name"]; // Output: Laptop
  

Common Array Functions

Worked Example: Book Search Catalog (Like Daraz)

$books = array(
    array("title" => "PHP for Beginners", "author" => "John Doe", "price" => 1500),
    array("title" => "Web Design", "author" => "Jane Smith", "price" => 2000)
);

// Search by title
function searchBooks($books, $keyword) {
    $results = array();
    foreach ($books as $book) {
        if (stripos($book["title"], $keyword) !== false) {
            array_push($results, $book);
        }
    }
    return $results;
}

$results = searchBooks($books, "PHP");
print_r($results);

Output:

Array ( [0] => Array ( [title] => PHP for Beginners ... ) )

4. Functions in PHP

Functions encapsulate reusable code. Syntax:

function functionName($param1, $param2) {
    // Code
    return $result;
}

Example: Product Discount Calculator

function calculateDiscount($price, $discountPercent) {
    $discount = $price * ($discountPercent / 100);
    return $price - $discount;
}

$finalPrice = calculateDiscount(5000, 10); // 4500
echo "Discounted Price: $" . $finalPrice;

Predefined Functions

5. Object-Oriented Programming (OOP) in PHP

OOP models real-world entities as classes (blueprints) and objects (instances).

classDiagram
    class User {
        +String name
        +String email
        +greet() String
    }
    class Admin {
        +String role
        +banUser() void
    }
    User <|-- Admin
    note for User "Example: $user = new User('Rohan', 'rohan@example.com');"

Inheritance: Admin extends User (e.g., Daraz seller dashboard)

Key Concepts

1. Class: Template for objects.

   class User {
       // Properties
       public $name;
       public $email;
   
       // Method
       public function greet() {
           return "Hello, " . $this->name;
       }
   }
   

2. Object: Instance of a class.

   $user = new User();
   $user->name = "Rohan";
   $user->email = "rohan@example.com";
   echo $user->greet(); // Output: Hello, Rohan
   

3. Constructor: Special method to initialize objects.

   class User {
       public function __construct($name, $email) {
           $this->name = $name;
           $this->email = $email;
       }
   }
   $user = new User("Rohan", "rohan@example.com");
   

Inheritance

• Parent class: Base class (e.g., Animal).
• Child class: Inherits properties/methods (e.g., Dog extends Animal).

  class Animal {
      public function eat() {
          return "Eating...";
      }
  }
  
  class Dog extends Animal {
      public function bark() {
          return "Barking!";
      }
  }
  
  $dog = new Dog();
  echo $dog->eat(); // Output: Eating...
  echo $dog->bark(); // Output: Barking!
  

Real-World Example: eSewa User System

classDiagram
    class User {
        +String name
        +String email
        +greet() String
    }
    class Admin {
        +approvePayment()
    }
    User <|-- Admin

• User class: Stores name/email, provides greet().
• Admin class: Extends User and adds approvePayment() for eSewa transactions.

6. Cookies and Sessions

Cookies

• Small data stored on the client’s browser.
• Example: Remembering a user’s language preference.

  // Set cookie (expires in 1 hour)
  setcookie("language", "nepali", time() + 3600);
  
  // Retrieve cookie
  if (isset($_COOKIE["language"])) {
      echo "Language: " . $_COOKIE["language"];
  }
  

Sessions

• Server-side storage for user-specific data (e.g., shopping carts).
• Example: Pathao’s ride tracking.

  // Start session
  session_start();
  
  // Store data
  $_SESSION["cart"] = array("item1", "item2");
  
  // Retrieve data
  print_r($_SESSION["cart"]);
  

Comparison

7. JSON in PHP

JSON (JavaScript Object Notation) is a lightweight data format for APIs (e.g., Daraz’s product data).

PHP ↔ JSON Conversion

// Array to JSON
$data = array("name" => "Rohan", "age" => 20);
$json = json_encode($data);
echo $json; // Output: {"name":"Rohan","age":20}

// JSON to Array
$json = '{"name":"Rohan","age":20}';
$data = json_decode($json, true);
print_r($data); // Output: Array ( [name] => Rohan ... )

Example: API Response (Like NEPSE Stock Data)

$stockData = array(
    "symbol" => "NEPSE",
    "price" => 2500.50,
    "change" => "+1.2%"
);

echo json_encode($stockData);

Output:

{"symbol":"NEPSE","price":2500.5,"change":"+1.2%"}

In the Real World

1. eSewa (Nepal)

   • PHP + OOP: The backend uses PHP classes to model User, Transaction, and Payment objects. When you pay a bill, PHP validates inputs (e.g., account number) and processes the transaction via a database.
   • Cookies: Store your login status to avoid repeated authentication.

2. Daraz (Nepal)

   • Arrays: Product catalogs are stored as multidimensional arrays (e.g., $products[id][name][price]). PHP loops through these arrays to display search results.
   • JSON: API responses (e.g., product details) are sent as JSON, which PHP decodes into arrays for display.

3. Khalti (Nepal)

   • Sessions: Maintain your cart items across pages. When you checkout, PHP session data ($_SESSION["cart"]) is processed to generate an invoice.
   • Form Validation: PHP validates payment details (e.g., card number format) before processing.

4. NTC Website (Nepal)

   • Functions: Reusable PHP functions calculate electricity bills based on consumption tiers (e.g., calculateBill($units)).

Exam Tip

1. Form Validation: Always validate inputs on the server-side (PHP) even if JavaScript is used client-side. Example:

   if (empty($_POST["username"])) {
       echo "Username is required!";
   }
   

2. Arrays: Practice traversing and manipulating arrays (e.g., foreach, array_push). Examiners often ask for dynamic data handling.
3. OOP: Focus on:
   • Defining classes with properties/methods.
   • Creating objects using new.
   • Inheritance (extends) for code reuse.
4. JSON: Know how to encode/decode between PHP arrays and JSON strings. API-related questions are common.
5. Cookies/Sessions: Understand the difference and when to use each (e.g., sessions for sensitive data like carts).

Pro Tip: For exam questions on form validation, always:

1. Check if fields are empty (empty($_POST["field"])).
2. Validate email format (filter_var($email, FILTER_VALIDATE_EMAIL)).
3. Sanitize inputs (htmlspecialchars()) to prevent XSS attacks.

In the real world

• Daraz (e-commerce) uses PHP arrays to store product catalogs (multidimensional arrays) and JSON for API responses (e.g., search results).
• eSewa (payment gateway) relies on PHP sessions to track user logins and maintain cart data across pages.
• Nepali banks (e.g., NMB, Global IME) use OOP in PHP to model Account, Transaction, and Customer classes for secure banking operations.