contact_form_submit.php

<?php

    require_once("db_connection.php");

    $errors = [];
    $response = [];

    if(isset($_POST['submit'])) {
        // Get the form data
        $fullname = $con->real_escape_string($_POST['fullname']);
        $email = $con->real_escape_string($_POST['email']);
        $subject = $con->real_escape_string($_POST['subject']);
        $message = $con->real_escape_string($_POST['message']);

        // Validate form fields
        if(empty($fullname)) {
            $errors[] = "Please enter your name.";
        }

        if(empty($email) || filter_var($email, FILTER_VALIDATE_EMAIL) === false) {
            $errors[] = "Please enter a valid email.";
        }

        if(empty($subject)) {
            $errors[] = "Please enter subject.";
        }

        if(empty($message)) {
            $errors[] = "Please enter your message.";
        }

        // Check errorss
        if(empty($errors)) {

            $sql="INSERT INTO contact_form_data (fullname, email, subject, message) VALUES ('".$fullname."','".$email."', '".$subject."', '".$message."')";

            if(!$result = $con->query($sql)){
                $errors[] = 'There was an error running the query <br>' . $con->error;
            } else {
                require_once("send_mail.php");
                $response[] = "Thank you! We will contact you soon";
            }

        }

    }
?>