What best practices should be followed when modifying PHP code to include interactive elements like dropdown menus in contact forms?

When modifying PHP code to include interactive elements like dropdown menus in contact forms, it is important to ensure proper validation and sanitization of user input to prevent security vulnerabilities such as SQL injection or cross-site scripting attacks. Additionally, the code should be structured in a way that promotes maintainability and scalability, such as separating the form logic from the display logic using templates or classes.

<form method="post" action="submit_form.php">
    <label for="dropdown">Select Option:</label>
    <select name="dropdown" id="dropdown">
        <option value="option1">Option 1</option>
        <option value="option2">Option 2</option>
        <option value="option3">Option 3</option>
    </select>
    <input type="submit" value="Submit">
</form>