What are the potential security risks associated with implementing custom PHP code in an online shop environment?

Potential security risks associated with implementing custom PHP code in an online shop environment include SQL injection, cross-site scripting (XSS) attacks, insecure file uploads, and inadequate input validation. To mitigate these risks, it is crucial to sanitize user input, use parameterized queries for database interactions, validate and filter file uploads, and implement secure coding practices.

// Example of using parameterized queries to prevent SQL injection
$stmt = $pdo->prepare('SELECT * FROM users WHERE username = :username');
$stmt->execute(['username' => $username]);