What are the potential pitfalls of using multiple queries within a PHP script to retrieve data from a database?

Using multiple queries within a PHP script to retrieve data from a database can lead to performance issues, increased server load, and potential security vulnerabilities such as SQL injection attacks. To mitigate these risks, it is recommended to use prepared statements and parameterized queries to prevent SQL injection, and to optimize your queries to minimize the number of database calls.

// Example of using prepared statements to retrieve data from a database in a single query

// Connect to the database
$pdo = new PDO('mysql:host=localhost;dbname=mydatabase', 'username', 'password');

// Prepare a statement
$stmt = $pdo->prepare('SELECT * FROM users WHERE id = :id');

// Bind parameters
$stmt->bindParam(':id', $userId);

// Execute the query
$stmt->execute();

// Fetch the results
$user = $stmt->fetch(PDO::FETCH_ASSOC);

// Use the retrieved data
echo $user['username'];