How can developers ensure a seamless transition from static HTML to dynamic PHP content without compromising security or user experience?

To ensure a seamless transition from static HTML to dynamic PHP content without compromising security or user experience, developers can use PHP's built-in functions for input validation and output escaping. By properly sanitizing user input and escaping output, developers can prevent common security vulnerabilities such as cross-site scripting (XSS) attacks. Additionally, developers should follow best practices for secure coding, such as using prepared statements for database queries and implementing proper error handling.

// Example of sanitizing user input and escaping output
$user_input = $_POST['user_input'];
$clean_input = htmlspecialchars(strip_tags($user_input));
echo $clean_input;