Using PHP and MySQL to build web pages

	<?php // sqltest.php
		require_once 'login.php';
	
		try {
			// Establish a database connection using PDO
			$pdo = new PDO($attr, $user, $pass, $opts);
		} catch (PDOException $e) {
			// Generic error handling
			echo "Database connection error.";
			exit;
		}
	
		// Function to validate ISBN
		function validate_isbn($isbn) {
			return preg_match('/^\d{10}$|^\d{13}$/', $isbn); // Match 10 or 13 digits
		}
	
		// Function to check if any field is empty
		function are_fields_empty(...$fields) {
			foreach ($fields as $field) {
				if (empty(trim($field))) {
					return true; // Return true if any field is empty
				}
			}
			return false; // Return false if all fields are filled
		}
	
		// Function to validate year
		function validate_year($year) {
			return preg_match('/^\d{4}$/', $year) && $year > 0; // Checks if year is a 4-digit positive integer
		}
	
		// Initialize an array to hold error messages
		$errors = [];
	
		// Handle deletion of records
		if (isset($_POST['delete']) && isset($_POST['isbn'])) {
			$isbn = get_post($pdo, 'isbn');
			// Use prepared statements to execute the deletion
			$stmt = $pdo->prepare("DELETE FROM classics WHERE isbn = :isbn");
			$stmt->execute(['isbn' => $isbn]);
		}
	
		// Handle insertion of records
		if ($_SERVER["REQUEST_METHOD"] == "POST") {
			$author = get_post($pdo, 'author');
			$title = get_post($pdo, 'title');
			$category = get_post($pdo, 'category');
			$year = get_post($pdo, 'year');
			$isbn = get_post($pdo, 'isbn');
	
			// Validate fields
			if (are_fields_empty($author, $title, $category, $year, $isbn)) {
				$errors[] = "All fields are required!";
			}
			if (!validate_isbn($isbn)) {
				$errors[] = "ISBN must be either 10 or 13 digits!";
			}
			if (!validate_year($year)) {
				$errors[] = "Year must be a valid 4-digit integer!";
			}
	
			// Check for duplicate ISBN
			if (empty($errors)) {
				$stmt = $pdo->prepare("SELECT COUNT(*) FROM classics WHERE isbn = :isbn");
				$stmt->execute(['isbn' => $isbn]);
				$exists = $stmt->fetchColumn();
				
				if ($exists) {
					$errors[] = "This ISBN already exists in the database!";
				}
			}
	
			// If no errors, insert the record
			if (empty($errors)) {
				// Use prepared statements for insertion
				$stmt = $pdo->prepare("INSERT INTO classics (author, title, category, year, isbn) VALUES (:author, :title, :category, :year, :isbn)");
				$stmt->execute(['author' => $author, 'title' => $title, 'category' => $category, 'year' => $year, 'isbn' => $isbn]);
			}
		}
	
		// Display error messages if any
		if (!empty($errors)) {
			echo "<section style='color:red;'>";
			foreach ($errors as $error) {
				echo "<p>$error</p>";
			}
			echo "</section>";
		}
	
		// Pre-fill form values to retain user input
		$form_author = $_POST['author'] ?? '';
		$form_title = $_POST['title'] ?? '';
		$form_category = $_POST['category'] ?? '';
		$form_year = $_POST['year'] ?? '';
		$form_isbn = $_POST['isbn'] ?? '';
	
		echo <<<_END
		<form action="sqltest.php" method="post"><pre>
			Author <input type="text" name="author" value="$form_author">
			 Title <input type="text" name="title" value="$form_title">
		  Category <input type="text" name="category" value="$form_category">
			  Year <input type="text" name="year" value="$form_year">
			  ISBN <input type="text" name="isbn" value="$form_isbn">
				   <input type="submit" value="ADD RECORD">
		</pre></form>
	_END;
	
		// Fetch and display records from the database
		$query = "SELECT * FROM classics";
		$result = $pdo->query($query);
	
		while ($row = $result->fetch()) {
			$r0 = htmlspecialchars($row['author']);
			$r1 = htmlspecialchars($row['title']);
			$r2 = htmlspecialchars($row['category']);
			$r3 = htmlspecialchars($row['year']);
			$r4 = htmlspecialchars($row['isbn']);
	
			echo <<<_END
		<pre>
			Author: $r0
			 Title: $r1
		  Category: $r2
			  Year: $r3
			  ISBN: $r4
		</pre>
		<form action='sqltest.php' method='post'>
			<input type='hidden' name='delete' value='yes'>
			<input type='hidden' name='isbn' value='$r4'>
			<input type='submit' value='DELETE RECORD'></form>
	_END;
		}
	
		function get_post($pdo, $var) {
			return htmlspecialchars(trim($_POST[$var] ?? ''));
		}
	?>