What are the potential drawbacks of relying solely on client-side validation for form completeness?

Relying solely on client-side validation for form completeness can be risky as it can be easily bypassed by users who disable JavaScript or manipulate the code. To ensure data integrity, it is important to also perform server-side validation. This involves checking the submitted data on the server before processing it further.

if ($_SERVER["REQUEST_METHOD"] == "POST") {
    // Perform server-side validation
    if (empty($_POST["username"]) || empty($_POST["password"])) {
        echo "Please fill in all required fields.";
    } else {
        // Process the form data
    }
}