PHP and MySQLi Tutorial

Variables are containers for storing information. In PHP, a variable starts with the $ sign, followed by the name of the variable. PHP supports String, Integer, Float, Boolean, Array, Object, and NULL data types.

<?php
$name = "Alice";      // String
$age = 30;            // Integer
$height = 5.8;        // Float
$isStudent = true;    // Boolean
$hobbies = ["reading", "hiking"]; // Array
// var_dump() shows the type and value
var_dump($name);
var_dump($age);
?>

Operators are used to perform operations on variables and values. Common types include Arithmetic (+, -, *, /), Assignment (=, +=), Comparison (==, ===, !=), and Logical (&&, ||, !).

<?php
$x = 10;
$y = 5;

// Arithmetic
echo $x + $y; // 15

// Comparison
var_dump($x > $y); // bool(true)

// Logical
$isLoggedIn = true;
$hasPermission = false;
var_dump($isLoggedIn && $hasPermission); // bool(false)
?>

Conditional statements allow you to perform different actions based on different conditions. if, elseif, and else are the primary tools for this.

<?php
$grade = 85;

if ($grade >= 90) {
    echo "Excellent!";
} elseif ($grade >= 75) {
    echo "Good job!";
} else {
    echo "Please review the material.";
}
?>

Loops are used to execute the same block of code a specified number of times or while a certain condition is true. Types include for, while, do-while, and foreach (for arrays).

<?php
// for loop
for ($i = 1; $i <= 3; $i++) {
    echo "Number $i <br>";
}

// foreach loop
$colors = ["red", "green", "blue"];
foreach ($colors as $color) {
    echo "$color <br>";
}
?>

A function is a block of code that can be reused multiple times. You can define your own functions to perform specific tasks.

<?php
function greetUser($name) {
    return "Hello, " . $name . "!";
}

echo greetUser("Bob"); // Outputs: Hello, Bob!
?>

Arrays store multiple values in one single variable. PHP supports indexed arrays (numeric keys), associative arrays (named keys), and multidimensional arrays (arrays containing other arrays).

<?php
// Indexed array
$cars = ["Volvo", "BMW", "Toyota"];
echo $cars[1]; // BMW

// Associative array
$user = [
    "name" => "John Doe",
    "email" => "john@example.com"
];
echo $user["email"]; // john@example.com
?>

Superglobals are built-in variables that are always available in all scopes. They provide information about the request and server environment.

• $_GET: Contains data sent in the URL query string.
• $_POST: Contains data sent via an HTTP POST request (e.g., from a form).
• $_SERVER: Contains information about the server and execution environment.
• $_SESSION: Contains session variables.
• $_COOKIE: Contains HTTP cookies.

<?php
// If URL is example.com?name=David
echo $_GET['name']; // Outputs: David

// Shows the server's host name
echo $_SERVER['HTTP_HOST'];
?>

PHP is excellent at handling HTML forms. You can collect data from form fields using the $_GET or $_POST superglobals, depending on the form's `method` attribute.

<!-- HTML Form -->
<form action="process.php" method="post">
    Name: <input type="text" name="username">
    <input type="submit">
</form>

<?php
// In process.php
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    // Collect and sanitize the input
    $name = htmlspecialchars($_POST['username']);
    echo "Hello, " . $name;
}
?>

PHP has a rich set of functions for working with strings.

<?php
$sentence = "The quick brown fox";

// Get string length
echo strlen($sentence); // 19

// Find position of a word
echo strpos($sentence, "brown"); // 10

// Replace text
echo str_replace("fox", "dog", $sentence); // The quick brown dog
?>

PHP can create, read, write to, and close files on the server.

<?php
$file = 'data.txt';

// Write to a file (will create if it doesn't exist)
file_put_contents($file, "Hello World!");

// Read from a file
$content = file_get_contents($file);
echo $content; // Hello World!
?>

Basic error handling can be done with the die() or exit() functions, which stop script execution and display a message. This is often used for critical errors, like a failed database connection.

<?php
$file = 'non_existent_file.txt';
$handle = fopen($file, 'r') or die("Could not open file!");

// More advanced error handling uses try...catch blocks
// for exceptions, which is common in modern PHP.
?>

You can include the content of one PHP file into another. require will produce a fatal error and stop the script if the file is not found, while include will only produce a warning and continue.

<!-- In index.php -->
<?php
// Use require for essential files like database connections.
require 'config.php'; 

// Use include for non-critical templates.
include 'header.php'; 
?>

OOP allows you to structure your code around objects. A class is a blueprint for objects, and an object is an instance of a class. Classes have properties (variables) and methods (functions).

<?php
class Car {
    // Property
    public $color;

    // Method
    public function startEngine() {
        return "Engine started!";
    }
}

// Create an object
$myCar = new Car();
$myCar->color = "red";

echo $myCar->color; // red
echo $myCar->startEngine(); // Engine started!
?>

Cookies are stored on the user's browser, while Sessions are stored on the server. They are both used to store user information across multiple page visits, like login status.

<?php
// Start a session (must be at the very top of the script)
session_start();

// Set a session variable
$_SESSION["username"] = "admin";

// Access it on another page
echo "Welcome, " . $_SESSION["username"];

// Set a cookie that expires in 1 hour
setcookie("user_preference", "dark_mode", time() + 3600);
?>

The date() function is used to format a date and/or a time.

<?php
// Set the default timezone to avoid warnings
date_default_timezone_set('America/New_York');

// Y for 4-digit year, m for month, d for day
echo "Today is " . date("Y-m-d") . "<br>";

// h:i:s a for hours, minutes, seconds, and am/pm
echo "The time is " . date("h:i:s a");
?>

To interact with a database, you must first establish a connection. The MySQLi extension provides an object-oriented way to do this. You need the server name, username, password, and database name.

<?php
$servername = "localhost";
$username = "your_username";
$password = "your_password";
$dbname = "your_database";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);

// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}
echo "Connected successfully";
?>

Once connected, you can execute SQL queries using the query() method. For queries that return data (like SELECT), it returns a result object. For others (INSERT, UPDATE), it returns true on success or false on failure.

<?php
// Assumes $conn is an active connection object
$sql = "SELECT id, firstname, lastname FROM Guests";
$result = $conn->query($sql);

if ($result) {
    echo "Query executed successfully!";
} else {
    echo "Error: " . $conn->error;
}
?>

After a SELECT query, you need to fetch the data from the result object. The most common method is fetch_assoc(), which returns one row at a time as an associative array.

<?php
// Assumes $result is a valid result object
if ($result->num_rows > 0) {
    // Loop through each row
    while($row = $result->fetch_assoc()) {
        echo "ID: " . $row["id"] . " - Name: " . $row["firstname"] . "<br>";
    }
} else {
    echo "0 results found";
}
?>

Prepared statements are a crucial security feature. They prevent SQL injection attacks by separating the SQL command from the data. You use placeholders (?) for data, which is later bound to the query.

<?php
// Prepare the statement
$stmt = $conn->prepare("INSERT INTO Guests (firstname, email) VALUES (?, ?)");

// Bind parameters: 'ss' means two strings
$stmt->bind_param("ss", $firstname, $email);

// Set parameters and execute
$firstname = "Mary";
$email = "mary@example.com";
$stmt->execute();

echo "New record created successfully";

$stmt->close();
?>

It's good practice to close the database connection when you're finished with it to free up resources.

<?php
// Assumes $conn is an active connection object
$conn->close();
echo "Connection closed.";
?>