PHP Basics & Syntax: Variables, Control Flow, Functions & Output

TAKEAWAYS:

• PHP is a server-side scripting language embedded in HTML to generate dynamic content.
• Variables in PHP start with $ and support dynamic typing (no declaration needed).
• Control structures (if, for, while) enable logic branching and iteration.
• Functions modularize code; PHP supports both built-in and user-defined functions.
• Output is generated via echo/print or HTML templates; error handling uses error_get_last().
• Real-world use: Khalti’s payment validation, Daraz’s order processing, and Ncell’s user session tracking rely on PHP’s syntax.

1. Introduction to PHP

PHP (Hypertext Preprocessor) is an open-source, server-side scripting language designed for web development. It processes logic on the server, generates dynamic HTML, and interacts with databases. Unlike client-side languages (JavaScript), PHP runs before the page is sent to the browser.

Key Features:

• Embedded in HTML: <php echo "Hello"; ?>
• Cross-platform: Works on Linux, Windows, macOS.
• Database integration: MySQL, PostgreSQL, SQLite.
• Extensible: Supports OOP and procedural programming.

2. PHP Syntax Basics

PHP code is enclosed in <?php ... ?> tags. Below are core syntax elements:

Variables

• Variables start with $ (e.g., $name).
• Case-sensitive: $Name ≠ $name.
• Dynamic typing: No need to declare types.

flowchart TD
    A["$name = 'Alice'"] --> B["$name = 10"] --> C["$name = true"]
    C -->|"PHP allows re-assignment"| D["No type errors"]

Data Types

PHP supports 8 primary data types:

3. Output in PHP

PHP generates output using:

• echo: Displays variables/strings (no semicolon needed).
• print: Similar to echo but returns 1.
• var_dump(): Debugs variable types/values.

Example: Dynamic HTML Output

<?php
$name = "John";
echo "<h1>Hello, $name!</h1>"; // Output: <h1>Hello, John!</h1>
?>

4. Control Structures

PHP uses if, else, switch, for, while, and foreach for logic.

Conditional Statements

flowchart TD
    A["Start"] --> B{"age >= 18?"}
    B -->|"Yes"| C["Adult\n(age ≥ 18)"]
    B -->|"No"| D["Minor\n(age < 18)"]
    C --> E["Can vote"]
    D --> F["Cannot vote"]

Example: Age Validation

<?php
$age = 20;
if ($age >= 18) {
    echo "Eligible to vote.";
} else {
    echo "Too young.";
}
?>

Output: Eligible to vote.

Loops

• for: Fixed iterations.
• while: Runs while condition is true.
• foreach: Iterates arrays.

Example: Print Numbers 1–5

<?php
for ($i = 1; $i <= 5; $i++) {
    echo "$i ";
}
?>

Output: 1 2 3 4 5

5. Functions

Functions encapsulate reusable logic. PHP has built-in functions (e.g., date(), rand()) and user-defined ones.

User-Defined Functions

<?php
function greet($name) {
    return "Hello, $name!";
}
echo greet("Alice"); // Output: Hello, Alice!
?>

Built-in Functions

Example: Generate Unique ID

<?php
function generateID() {
    return "ID-" . rand(1000, 9999);
}
echo generateID(); // Example: ID-4567
?>

6. Arrays in PHP

Arrays store multiple values in a single variable. Types:

• Indexed: Ordered (e.g., $fruits[0] = "Apple").
• Associative: Key-value pairs (e.g., $books["title"] = "PHP Guide").

Example: Associative Array (Books)

<?php
$books = [
    "title1" => "PHP Basics",
    "title2" => "Advanced PHP"
];
foreach ($books as $title) {
    echo "$title<br>";
}
?>

Output:

PHP Basics
Advanced PHP

7. File Handling

PHP reads/writes files using fopen(), fread(), fwrite().

Example: Save User Data to File

<?php
$name = $_POST['name'];
$age = $_POST['age'];
$file = fopen("users.txt", "a");
fwrite($file, "$name, $age\n");
fclose($file);
?>

8. Error Handling

PHP logs errors via error_get_last() or ini_set('display_errors', 1).

Example: Catch Errors

<?php
try {
    $result = 10 / 0; // Division by zero
} catch (Exception $e) {
    echo "Error: " . $e->getMessage();
}
?>

Output: Error: Division by zero

In the Real World

1. Khalti’s Payment Validation

   • Idea Used: PHP’s if-else and POST data handling.
   • How: When a user submits payment details, Khalti’s backend uses PHP to validate fields (e.g., card number format) before processing the transaction. If invalid, it shows an error like:

     if (!preg_match("/^\d{16}$/", $cardNumber)) {
         echo "Invalid card number!";
     }
     

2. Daraz’s Order Processing

   • Idea Used: PHP arrays and loops.
   • How: Daraz stores order items in an associative array:

     $order = [
         "item1" => ["name" => "Laptop", "price" => 20000],
         "item2" => ["name" => "Mouse", "price" => 500]
     ];
     $total = array_sum(array_map(fn($item) => $item["price"], $order));
     

     The loop calculates the total price dynamically.

3. Ncell’s User Session Tracking

   • Idea Used: PHP sessions (session_start()).
   • How: When you log into Ncell’s app, PHP creates a session ID:

     session_start();
     $_SESSION['user'] = "user123";
     

     This ID persists across pages, enabling personalized dashboards.

Worked Example: Date Difference Calculator

Problem: Calculate the difference between two dates (e.g., 2023-10-01 and 2023-10-15).

Solution:

<?php
$date1 = "2023-10-01";
$date2 = "2023-10-15";
$diff = strtotime($date2) - strtotime($date1);
echo "Difference: " . round($diff / (60 * 60 * 24)) . " days";
?>

Output: Difference: 14 days

Comparison Table: PHP vs. JavaScript

Advantages/Disadvantages

Exam Tip

• Focus on:
  • Variable declaration and data types (e.g., $name = "Alice";).
  • Control structures (if, for, while) with traces.
  • Functions: Define and call (e.g., function add($a, $b) { return $a + $b; }).
  • File handling: fopen(), fwrite() for saving data.
  • Error handling: Use try-catch or error_get_last().
• Common Pitfalls:
  • Forgetting $ before variables (e.g., $name vs. name).
  • Incorrect array syntax (e.g., $arr[0] vs. $arr{0}).
  • Mixing PHP and HTML tags (e.g., <?php echo "<h1>"; ?>).
• Past Exam Patterns:
  • 50%: Write a PHP script for a given task (e.g., file handling, validation).
  • 30%: Explain syntax (e.g., "How to create a 2D array?").
  • 20%: Debug errors (e.g., "Fix this code to display an error message").

Visual Recap:

flowchart TD
    A["PHP Script"] --> B["Server Processes\n(Interpreter)"]
    B --> C["Generates HTML/CSS\n(Dynamic Content)"]
    C --> D["Browser Renders\n(Client-Side)"]
    D --> E["Dynamic Web Page"]
    C -->|"Also"| F["JSON/XML\n(API Responses)"]