What best practices should be followed when modifying PHP code to handle cases where certain variables may not be set, such as in the absence of search engine referrals?

When modifying PHP code to handle cases where certain variables may not be set, such as in the absence of search engine referrals, it is best practice to check if the variable is set using isset() or empty() functions before using it in your code. This helps prevent errors or warnings from being thrown when the variable is not set. You can also provide a default value or handle the case where the variable is not set gracefully to ensure the smooth functioning of your code.

// Check if the variable is set and assign a default value if not
$search_query = isset($_GET['q']) ? $_GET['q'] : '';

// Use the variable in your code
echo "Search query: " . $search_query;