Introduction to PHP Exercise Solutions | KalviGavani

(a) Different (b) Same (c) Error (d) 0

PHP (Hypertext Pre-Processor) is an open-source, server-side scripting language designed primarily for web development. It is embedded within HTML and processed on the server to generate dynamic web pages.

Key Features:

• Server-side execution — PHP code runs on the server, not in the browser
• Open source — free to use and distribute
• Cross-platform — runs on Windows, Linux, macOS
• Embedded in HTML — PHP code can be mixed with HTML using <?php ... ?> tags
• Database support — works with MySQL, PostgreSQL, SQLite, Oracle, and more
• Loosely typed — no need to declare variable types
• Large community — extensive documentation and community support
• Fast — efficient execution on web servers

Simple PHP example:

<?php
  echo "Hello, World!";
?>

PHP code is embedded in HTML using the <?php ... ?> tags. The server processes only the PHP portions and sends the resulting HTML to the browser.

<!DOCTYPE html>
<html>
<head>
    <title>PHP in HTML</title>
</head>
<body>
    <h1>Welcome to PHP</h1>
    <?php
        $name = "Arun";
        echo "<p>Hello, " . $name . "!</p>";
    ?>
    <p>This is static HTML content.</p>
    <?php
        $age = 20;
        echo "<p>Age: " . $age . "</p>";
    ?>
</body>
</html>

Output in browser:

The browser receives only HTML — the PHP code is processed and replaced on the server before the page is sent.

PHP variables are containers for storing data values. They are declared with the $ symbol followed by the variable name.

Rules for naming PHP variables: 1. Must start with $ followed by a letter or underscore 2. Cannot start with a number after $ 3. Can contain letters, numbers, and underscores 4. Are case-sensitive ($name and $Name are different) 5. Cannot contain spaces 6. Should not use PHP reserved keywords

<?php
  // Valid variable names
  $name = "Arun";
  $_age = 20;
  $student_name = "Bharathi";
  $marks2 = 85;

  // Invalid variable names
  // $1st_place = "Gold";    // Cannot start with number
  // $student name = "Arun"; // Cannot contain space
?>

PHP is loosely typed — the same variable can hold different data types:

<?php
  $x = 10;       // $x is Integer
  $x = 3.14;     // $x is now Float
  $x = "Hello";  // $x is now String
?>

PHP supports 8 data types, divided into 3 categories:

Scalar Types (simple values):

1. Integer — whole numbers without decimals

   $age = 20;
   $count = -5;

2. Float (Double) — numbers with decimal points

   $price = 99.50;
   $pi = 3.14159;

3. String — sequence of characters in quotes

   $name = "Arun";
   $msg = 'Hello World';

4. Boolean — logical value, true or false

   $is_student = true;
   $has_passed = false;

Compound Types (collections):

5. Array — ordered collection of values

   $colors = array("Red", "Green", "Blue");
   $scores = [85, 92, 78];

6. Object — instance of a class

   class Student {
       public $name;
   }
   $s = new Student();

Special Types:

7. NULL — variable with no value

   $result = NULL;

8. Resource — reference to external resource (e.g., database connection)

   $conn = mysqli_connect("localhost", "root", "");

<?php
  // echo with multiple arguments
  echo "Hello", " ", "World";  // Output: Hello World

  // print with single argument
  print "Hello World";          // Output: Hello World

  // print returns a value (can be used in expressions)
  $result = print "Hi";         // Output: Hi, $result = 1
?>

Arithmetic Operators:

<?php
  $a = 15;
  $b = 4;
  echo $a + $b;  // 19 (Addition)
  echo $a - $b;  // 11 (Subtraction)
  echo $a * $b;  // 60 (Multiplication)
  echo $a / $b;  // 3.75 (Division)
  echo $a % $b;  // 3 (Modulus - remainder)
?>

Comparison Operators:

<?php
  $x = 10;
  $y = "10";
  var_dump($x == $y);   // bool(true)  — same value
  var_dump($x === $y);  // bool(false) — different types
?>

PHP is a server-side scripting language — the code is executed on the web server, not on the client (browser). Here is how it works:

Step-by-step working: 1. The user types a URL or clicks a link that requests a .php file 2. The browser sends an HTTP request to the web server 3. The web server (Apache/Nginx) recognises the .php extension 4. The server passes the PHP file to the PHP interpreter (PHP engine) 5. The PHP interpreter processes the PHP code between <?php ?> tags 6. PHP may connect to a database (MySQL) to fetch or store data 7. The PHP interpreter generates HTML output from the PHP code 8. The server sends the pure HTML response back to the browser 9. The browser renders the HTML page for the user

Request-Response Flow:

Key points:

• The client never sees the PHP source code — only the HTML output
• PHP can interact with databases, files, and external services on the server
• Dynamic content (user-specific, database-driven) is generated before the page reaches the browser
• The same PHP file can produce different HTML for different users

Example:

<?php
  // File: welcome.php
  $hour = date("H");
  if ($hour < "12") {
      echo "Good Morning!";
  } elseif ($hour < "18") {
      echo "Good Afternoon!";
  } else {
      echo "Good Evening!";
  }
?>

When a user visits welcome.php, the server determines the current hour and sends an appropriate greeting. The browser only sees "Good Morning!" or "Good Afternoon!" — never the PHP code.

PHP Variables:

Variables are containers for storing data. In PHP:

• Declared with $ followed by the variable name
• No type declaration needed (loosely typed)
• Value can be changed during execution
• Case-sensitive

<?php
  $name = "Arun";        // String
  $age = 20;             // Integer
  $height = 5.8;         // Float
  $is_student = true;   // Boolean

  // Variable values can be changed
  $age = 21;             // $age is now 21

  // Variable interpolation in double quotes
  echo "My name is $name and I am $age years old.";
?>

Variable Scope:

• Local — declared inside a function, accessible only within that function
• Global — declared outside functions, accessible anywhere (use global keyword inside functions)
• Static — persists its value between function calls (use static keyword)

<?php
  $x = 10;  // Global scope

  function test() {
      global $x;  // Access global variable inside function
      echo $x;
  }
?>

PHP Constants:

Constants are identifiers for fixed values that cannot be changed during execution.

• Defined using define() function or const keyword
• Do not start with $
• Automatically global (accessible anywhere in the script)
• Cannot be redefined or undefined once set

<?php
  // Using define()
  define("SITE_NAME", "KalviGavani");
  define("PI", 3.14159);

  // Using const keyword
  const MAX_STUDENTS = 40;
  const VERSION = "2.0";

  echo SITE_NAME;    // Output: KalviGavani
  echo PI;           // Output: 3.14159
  echo MAX_STUDENTS; // Output: 40

  // Constants cannot be changed
  // PI = 3.14;      // ERROR! Cannot redefine constant
?>

Differences between Variables and Constants:

PHP operators are symbols that perform operations on variables and values.

Operator Precedence (high to low): 1. ++, -- (increment/decrement) 2. *, /, % (multiplication, division, modulus) 3. +, - (addition, subtraction) 4. <, <=, >, >= (comparison) 5. ==, !=, ===, !== (equality) 6. && (logical AND)

8. =, +=, .= (assignment)

PHP can be embedded in HTML in several ways. The server processes the PHP portions and outputs the resulting HTML to the browser.

Example 1: Simple PHP output in HTML

<!DOCTYPE html>
<html>
<head>
    <title>PHP Example</title>
</head>
<body>
    <h1>Welcome</h1>
    <?php
        echo "<p>Today is " . date("d-m-Y") . "</p>";
    ?>
</body>
</html>

Example 2: Using PHP variables in HTML

<!DOCTYPE html>
<html>
<head>
    <title>Student Details</title>
</head>
<body>
    <?php
        $student_name = "Arun";
        $roll_no = 101;
        $marks = 85;
    ?>
    <h1>Student Report</h1>
    <p>Name: <?php echo $student_name; ?></p>
    <p>Roll No: <?php echo $roll_no; ?></p>
    <p>Marks: <?php echo $marks; ?></p>
</body>
</html>

Example 3: PHP conditional logic in HTML

<!DOCTYPE html>
<html>
<head>
    <title>Result</title>
</head>
<body>
    <h1>Exam Result</h1>
    <?php
        $marks = 75;
        if ($marks >= 35) {
            echo "<p style='color:green;'>Pass — Marks: $marks</p>";
        } else {
            echo "<p style='color:red;'>Fail — Marks: $marks</p>";
        }
    ?>
</body>
</html>

Example 4: PHP loop in HTML

<!DOCTYPE html>
<html>
<head>
    <title>Number Table</title>
</head>
<body>
    <h2>Number Table of 5</h2>
    <table border="1">
        <?php
            for ($i = 1; $i <= 10; $i++) {
                echo "<tr><td>5 x $i</td><td>" . (5 * $i) . "</td></tr>";
            }
        ?>
    </table>
</body>
</html>

Example 5: Shorthand PHP tags in HTML

<p>Name: <?= $student_name ?></p>
<!-- <?= ... ?> is shorthand for <?php echo ... ?> -->

Important points about embedding PHP in HTML:

• The file must have a .php extension (not .html)
• PHP code must be enclosed in <?php ... ?> tags
• The server must have PHP installed and configured
• The browser receives only HTML — PHP code is never visible to the user
• Multiple PHP blocks can be used in a single file
• Variables declared in one PHP block are accessible in subsequent blocks

Comments are non-executable lines in code that explain what the code does. They are ignored by the PHP interpreter.

Single-line Comments:

Used for short explanations. There are two syntaxes:

<?php
  // This is a single-line comment using double slash
  $name = "Arun";  // Comment after code

  # This is a single-line comment using hash
  $age = 20;       # Comment after code
?>

Both // and # make everything after them on the same line a comment. The // style is more commonly used and recommended.

Multi-line Comments:

Used for longer explanations or commenting out multiple lines of code.

<?php
  /*
   * This is a multi-line comment.
   * It can span multiple lines.
   * Useful for detailed explanations.
   */
  $name = "Arun";
  $age = 20;

  /* Multi-line comment can also be used
     to temporarily disable code blocks */
  /*
  $test = "This code won't run";
  echo $test;
  */
?>

When to use comments:

Best practices: 1. Use comments to explain why the code does something, not what it does 2. Keep comments concise and relevant 3. Update comments when you change code 4. Use multi-line comments for function/class documentation 5. Avoid over-commenting obvious code

<?php
  // Good comment: explains why
  $discount = 0.10;  // 10% discount for premium members

  // Unnecessary comment: states the obvious
  $total = $price + $tax;  // adds price and tax
?>