What are the implications of relying solely on client-side JavaScript for form validation in PHP applications, and how can server-side validation be implemented effectively?

Relying solely on client-side JavaScript for form validation in PHP applications can lead to security vulnerabilities as malicious users can bypass the client-side validation. To implement effective server-side validation, you should validate the form data on the server before processing it. This can be done by checking the submitted data against predefined rules and returning appropriate error messages if validation fails.

// Server-side validation example
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $name = $_POST["name"];
    $email = $_POST["email"];

    if (empty($name)) {
        $errors[] = "Name is required";
    }

    if (empty($email) || !filter_var($email, FILTER_VALIDATE_EMAIL)) {
        $errors[] = "Valid email is required";
    }

    if (!empty($errors)) {
        foreach ($errors as $error) {
            echo $error . "<br>";
        }
    } else {
        // Process the form data
    }
}