PHP Introduction

What is PHP?

PHP logo

PHP (Hypertext Preprocessor) is a widely-used open-source server-side scripting language specifically designed for web development. Originally created by Rasmus Lerdorf in 1994, PHP has evolved into one of the most popular programming languages for building dynamic websites and web applications.

Unlike client-side languages like JavaScript that run in the user's browser, PHP code runs on the web server, generating HTML that is then sent to the client. This server-side execution makes PHP powerful for creating dynamic content, processing form data, accessing databases, and managing sessions.

Why Learn PHP?

Here are some compelling reasons to learn PHP:

Widespread Usage: PHP powers approximately 77% of all websites with a known server-side language
Easy to Learn: PHP has a gentle learning curve, particularly for those with HTML knowledge
Versatility: It works on all major operating systems and supports most web servers
Extensive Libraries: PHP includes numerous built-in functions and extensions
Strong Community: Large community support with abundant documentation and resources
Job Opportunities: High demand for PHP developers in the job market
Popular Frameworks: Powers frameworks like Laravel, Symfony, and WordPress (which runs about 43% of all websites)

Setting Up PHP

Before writing PHP code, you need a proper development environment. Here's how to get started:

Option 1: Local Development Environment

You can set up PHP locally using one of these methods:

Install separate components: Install PHP, a web server (like Apache), and MySQL individually
All-in-one packages: Use packages like XAMPP, WAMP (Windows), MAMP (Mac), or LAMP (Linux) that bundle everything together

Option 2: Online Development Environment

If you prefer not to install anything, you can use online services like:

Replit
CodePen
JSFiddle (with PHP support)

PHP Basic Syntax

Let's start with basic PHP syntax. PHP code is enclosed within special start and end processing instructions:

php
<?php
  // Your PHP code goes here
  echo "Hello, World!";
?>

You can embed PHP code within HTML:

php
<!DOCTYPE html>
<html>
<head>
    <title>My First PHP Page</title>
</head>
<body>
    <h1>Welcome to PHP</h1>
    <?php
        echo "This is my first PHP script!";
    ?>
</body>
</html>

When executed, this produces:

html
<!DOCTYPE html>
<html>
<head>
    <title>My First PHP Page</title>
</head>
<body>
    <h1>Welcome to PHP</h1>
    This is my first PHP script!
</body>
</html>

Output with PHP

PHP uses several methods to output data:

echo: Outputs one or more strings
print: Similar to echo but can only output one string
print_r(): Used to print arrays in a readable format (useful for debugging)
var_dump(): Outputs information about a variable including its data type and value

Examples:

php
<?php
// Using echo
echo "Hello World!";
echo "Multiple ", "strings ", "can be ", "concatenated.";

// Using print
print "Hello World!";

// Outputting variables
$name = "John";
echo "Hello " . $name;  // Using concatenation
echo "Hello $name";     // Using variable inside a double-quoted string

// print_r and var_dump for debugging
$colors = array("red", "blue", "green");
print_r($colors);
var_dump($colors);
?>

Output:

Hello World!
Multiple strings can be concatenated.
Hello World!
Hello John
Hello John
Array ( [0] => red [1] => blue [2] => green )
array(3) { [0]=> string(3) "red" [1]=> string(4) "blue" [2]=> string(5) "green" }

PHP Variables

Variables in PHP are used to store data, like numbers, strings, arrays, and objects. Here's how to work with them:

Variable Declaration

PHP variables start with a dollar sign ($) followed by the variable name:

php
<?php
$text = "Hello World";
$number = 42;
$float = 3.14;
$isTrue = true;
$colors = array("red", "green", "blue");

// Displaying variables
echo $text;  // Outputs: Hello World
echo $number;  // Outputs: 42
?>

Variable Rules

Variable names must start with a letter or underscore
Variable names can only contain alphanumeric characters and underscores (A-z, 0-9, and _)
Variable names are case-sensitive ( $name and$ NAME are different variables)

Variable Scope

PHP has three different variable scopes:

Local: Variables declared within a function
Global: Variables declared outside a function
Static: Variables that don't lose their value when the function exits

php
<?php
$globalVar = "I'm a global variable";  // Global scope

function testFunction() {
    $localVar = "I'm a local variable";  // Local scope
    
    // To access global variables inside a function:
    global $globalVar;
    echo $globalVar;
    
    // Static variable example
    static $counter = 0;
    $counter++;
    echo "Function called $counter time(s)";
}

testFunction();
testFunction();  // The counter will be 2 this time
?>

PHP Data Types

PHP supports the following data types:

String: Text enclosed in quotes (e.g., "Hello" or 'World')
Integer: Whole numbers (e.g., 42, -7)
Float: Decimal numbers (e.g., 3.14, -2.5)
Boolean: True or false
Array: Store multiple values in a single variable
Object: Instance of a class
NULL: Special type representing a variable with no value
Resource: External resource reference (like a database connection)

php
<?php
// String
$string = "Hello World";

// Integer
$integer = 25;

// Float
$float = 3.14;

// Boolean
$isTrue = true;
$isFalse = false;

// Array
$simpleArray = array("Apple", "Banana", "Orange");
$associativeArray = array("name" => "John", "age" => 30);

// Object
class Person {
    public $name;
    public function __construct($name) {
        $this->name = $name;
    }
}
$person = new Person("John");

// NULL
$nullVar = NULL;

// Checking data types
var_dump($string);    // string(11) "Hello World"
var_dump($integer);   // int(25)
var_dump($isTrue);    // bool(true)
var_dump($simpleArray);  // array(3) { [0]=> string(5) "Apple" [1]=> string(6) "Banana" [2]=> string(6) "Orange" }
?>

PHP Constants

Constants are identifiers for values that cannot change during script execution:

php
<?php
// Define a constant
define("GREETING", "Hello World!");
echo GREETING;  // Outputs: Hello World!

// Constants are global and can be used anywhere in your script
function testFunction() {
    echo GREETING;  // Will work without any additional code
}

// PHP predefined constants
echo PHP_VERSION;  // Outputs the PHP version
echo __FILE__;     // Outputs the current file path
?>

PHP Operators

PHP provides various operators for performing operations on variables and values:

Arithmetic Operators

php
<?php
$a = 10;
$b = 3;

echo $a + $b;  // Addition: 13
echo $a - $b;  // Subtraction: 7
echo $a * $b;  // Multiplication: 30
echo $a / $b;  // Division: 3.33...
echo $a % $b;  // Modulus (remainder): 1
echo $a ** $b; // Exponentiation: 1000 (10^3)
?>

Assignment Operators

php
<?php
$x = 10;     // Basic assignment
$x += 5;     // Same as $x = $x + 5
$x -= 3;     // Same as $x = $x - 3
$x *= 2;     // Same as $x = $x * 2
$x /= 4;     // Same as $x = $x / 4
$x %= 3;     // Same as $x = $x % 3
?>

Comparison Operators

php
<?php
$a = 10;
$b = "10";

var_dump($a == $b);   // Equal (value): bool(true)
var_dump($a === $b);  // Identical (value and type): bool(false)
var_dump($a != $b);   // Not equal: bool(false)
var_dump($a !== $b);  // Not identical: bool(true)
var_dump($a > 5);     // Greater than: bool(true)
var_dump($a < 20);    // Less than: bool(true)
var_dump($a >= 10);   // Greater than or equal to: bool(true)
var_dump($a <= 10);   // Less than or equal to: bool(true)
?>

Logical Operators

php
<?php
$x = true;
$y = false;

var_dump($x && $y);  // AND: bool(false)
var_dump($x || $y);  // OR: bool(true)
var_dump(!$x);       // NOT: bool(false)
?>

PHP Control Structures

Control structures allow you to control the flow of your script:

If-Else Statement

php
<?php
$hour = 14;  // 2 PM

if ($hour < 12) {
    echo "Good Morning!";
} elseif ($hour < 18) {
    echo "Good Afternoon!";
} else {
    echo "Good Evening!";
}
// Output: Good Afternoon!
?>

Switch Statement

php
<?php
$dayOfWeek = "Monday";

switch ($dayOfWeek) {
    case "Monday":
        echo "Start of work week.";
        break;
    case "Friday":
        echo "End of work week.";
        break;
    case "Saturday":
    case "Sunday":
        echo "Weekend!";
        break;
    default:
        echo "Midweek.";
}
// Output: Start of work week.
?>

Loops

PHP provides several types of loops:

While Loop

php
<?php
$i = 1;
while ($i <= 5) {
    echo $i . " ";
    $i++;
}
// Output: 1 2 3 4 5
?>

Do-While Loop

php
<?php
$i = 1;
do {
    echo $i . " ";
    $i++;
} while ($i <= 5);
// Output: 1 2 3 4 5
?>

For Loop

php
<?php
for ($i = 1; $i <= 5; $i++) {
    echo $i . " ";
}
// Output: 1 2 3 4 5
?>

Foreach Loop

php
<?php
$colors = array("red", "green", "blue");

foreach ($colors as $color) {
    echo $color . " ";
}
// Output: red green blue

// With key and value
$person = array("name" => "John", "age" => 30, "job" => "Developer");
foreach ($person as $key => $value) {
    echo "$key: $value, ";
}
// Output: name: John, age: 30, job: Developer,
?>

A Real-World Example: PHP Form Handling

One of the most common uses of PHP is form handling. Here's a simple example:

php
<!DOCTYPE html>
<html>
<head>
    <title>PHP Form Example</title>
</head>
<body>
    <h2>Contact Form</h2>
    
    <?php
    // Define variables and set to empty values
    $nameErr = $emailErr = "";
    $name = $email = $message = "";
    $formSubmitted = false;
    
    // Form processing when submitted
    if ($_SERVER["REQUEST_METHOD"] == "POST") {
        $formSubmitted = true;
        
        // Validate name
        if (empty($_POST["name"])) {
            $nameErr = "Name is required";
        } else {
            $name = test_input($_POST["name"]);
        }
        
        // Validate email
        if (empty($_POST["email"])) {
            $emailErr = "Email is required";
        } else {
            $email = test_input($_POST["email"]);
            // Check if email address is well-formed
            if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
                $emailErr = "Invalid email format";
            }
        }
        
        // Get message
        $message = test_input($_POST["message"]);
    }
    
    // Function to sanitize input data
    function test_input($data) {
        $data = trim($data);
        $data = stripslashes($data);
        $data = htmlspecialchars($data);
        return $data;
    }
    ?>
    
    <!-- Show form if not submitted or if there are errors -->
    <?php if (!$formSubmitted || $nameErr || $emailErr): ?>
    
    <form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>">
        <div>
            <label for="name">Name:</label>
            <input type="text" name="name" value="<?php echo $name; ?>">
            <span style="color: red;"><?php echo $nameErr; ?></span>
        </div>
        
        <div>
            <label for="email">Email:</label>
            <input type="text" name="email" value="<?php echo $email; ?>">
            <span style="color: red;"><?php echo $emailErr; ?></span>
        </div>
        
        <div>
            <label for="message">Message:</label>
            <textarea name="message" rows="5" cols="40"><?php echo $message; ?></textarea>
        </div>
        
        <div>
            <input type="submit" name="submit" value="Submit">
        </div>
    </form>
    
    <?php else: ?>
    
    <!-- Thank you message when form is successfully submitted -->
    <div>
        <h3>Thank you for contacting us, <?php echo $name; ?>!</h3>
        <p>We have received your message and will respond to <?php echo $email; ?> soon.</p>
    </div>
    
    <?php endif; ?>
    
</body>
</html>

This example demonstrates:

Form creation and processing
Input validation
Security measures (sanitizing input)
Conditional output

PHP in Web Development

PHP's role in the web development process can be visualized as:

Summary

In this introduction, we've covered:

What PHP is and why it's important
Setting up a PHP development environment
Basic PHP syntax, including output methods
Variables and data types
Operators and control structures
A real-world example of form processing

PHP is a powerful language that forms the backbone of many web applications. As you continue your journey, you'll discover that PHP can handle everything from simple contact forms to complex e-commerce platforms and content management systems.

Exercises for Practice

Create a simple PHP script that displays your name and current date/time.
Build a form that collects a user's name, age, and favorite color, then displays this information.
Write a PHP script that determines if a number is even or odd.
Create a PHP page that displays a different message based on the time of day.
Build a simple calculator that can add, subtract, multiply, and divide two numbers.

Additional Resources

If you spot any mistakes on this website, please let me know at [email protected]. I’d greatly appreciate your feedback! :)

What is PHP?​

Why Learn PHP?​

Setting Up PHP​

Option 1: Local Development Environment​

Option 2: Online Development Environment​

PHP Basic Syntax​

Output with PHP​

PHP Variables​

Variable Declaration​

Variable Rules​

Variable Scope​

PHP Data Types​

PHP Constants​

PHP Operators​

Arithmetic Operators​

Assignment Operators​

Comparison Operators​

Logical Operators​

PHP Control Structures​

If-Else Statement​

Switch Statement​

Loops​

While Loop​

Do-While Loop​

For Loop​

Foreach Loop​

A Real-World Example: PHP Form Handling​

PHP in Web Development​

Summary​

Exercises for Practice​

Additional Resources​