How can the E-V-A principle help prevent header-related errors in PHP code?

The E-V-A principle (Escape, Validate, and Avoid) can help prevent header-related errors in PHP code by ensuring that user input is properly sanitized before being used in header functions. By escaping any user input using functions like htmlspecialchars(), validating input to ensure it meets expected criteria, and avoiding direct user input in header functions, developers can reduce the risk of header injection attacks and other vulnerabilities.

// Example of implementing the E-V-A principle to prevent header-related errors
$user_input = $_GET['user_input'];

// Validate the user input
if (is_numeric($user_input)) {
    // Escape the user input before using it in header functions
    $safe_user_input = htmlspecialchars($user_input);
    
    // Send a header with the sanitized user input
    header("Location: /page.php?user_input=$safe_user_input");
} else {
    // Handle invalid input
    echo "Invalid input";
}