How can database queries impact the functionality of PHP scripts in form validation?

Database queries can impact the functionality of PHP scripts in form validation by introducing potential security vulnerabilities such as SQL injection attacks. To mitigate this risk, developers should use parameterized queries or prepared statements to sanitize user input before executing any database queries. 

// Using parameterized queries to prevent SQL injection
$stmt = $pdo->prepare("SELECT * FROM users WHERE username = ?");
$stmt->execute([$username]);
$user = $stmt->fetch();

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database queries PHP scripts form validation functionality impact

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