Wordpress contact form without a plugin

wordpress-contact-form.php

<?php include("header.php"); ?>

<div class="page-container span_12 xs-md:p-0 pg">
    <section class="span_12 container grid">
        <main class="span_12 relative pg">

            <?php
            //response generation function
            
            $response = "";

            //function to generate response
            function my_contact_form_generate_response($type, $message){

            global $response;

            if($type == "success") $response = "<div class='pg border border_3 currentColor color_succes border-radius_5'><p class='color_inherit m_0'>{$message}</p></div>";
            else $response = "<div class='pg border border_3 currentColor color_error border-radius_5'><p class='color_inherit m_0'>{$message}</p></div>";
            }

            //response messages
            $missing_content = "Please supply all information.";
            $email_invalid = "Email address invalid.";
            $message_unsent = "Message was not sent. Try Again.";
            $message_sent = "Thanks! Your message has been sent.";

            //user posted variables
            $name = $_POST['message_name'];
            $email = $_POST['message_email'];
            $message = $_POST['message_text'];
            $human = $_POST['message_email'];

            //php mailer variables
            $to = get_option('admin_email');
            $subject = "Someone sent a message from ".get_bloginfo('name');
            $headers = 'From: '. $email . "\r\n" .
            'Reply-To: ' . $email . "\r\n";

            if(!$human == (strlen(trim($string)) > 0)){
            if($human == (strlen(trim($string)) > 1)) my_contact_form_generate_response("error", $not_human); //not human!
            else {

            //validate email
            if(!filter_var($email, FILTER_VALIDATE_EMAIL))
            my_contact_form_generate_response("error", $email_invalid);
            else //email is valid
            {
            //validate presence of name and message
            if(empty($name) || empty($message)){
            my_contact_form_generate_response("error", $missing_content);
            }
            else //ready to go!
            {
            $sent = wp_mail($to, $subject, strip_tags($message), $headers);
            if($sent) my_contact_form_generate_response("success", $message_sent); //message sent!
            else my_contact_form_generate_response("error", $message_unsent); //message wasn't sent
            }
            }
            }
            }
            else if ($_POST['submitted']) my_contact_form_generate_response("error", $missing_content);

            ?>

            <?php while ( have_posts() ) : the_post(); ?>

            <article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>

                <header class="text_center">
                    <h1><?php the_title(); ?></h1>
                </header>

                    <?php the_content(); ?>

                    <div class="span_12 grid row-gap" id="respond">
           
                        <form class="span_12 grid row-gap" action="<?php the_permalink(); ?>" method="post">

                            <div class="span_12 md-xl:span_6">
                                <label for="name">Name
                                <input type="text" name="message_name" value="<?php echo esc_attr($_POST['message_name']); ?>"> </label>
                            </div>

                            <div class="span_12 md-xl:span_6">
                                <label for="message_email">Email
                                <input type="text" name="message_email" value="<?php echo esc_attr($_POST['message_email']); ?>"></label>
                            </div>

                            <div class="span_12">
                                <label for="message_text">Message</label>
                                <textarea class="js__textarea" type="text" name="message_text"><?php echo esc_textarea($_POST['message_text']); ?></textarea>
                            </div>

                            <input type="hidden" name="submitted" value="1">
                            
                            <div class="span_12">
                                <?php echo $response; ?>
                            </div>
                            
                            <div class="span_12">
                                <input class="float_right w_auto" type="submit"></p>
                            </div>
                        </form>
                    </div>

            </article><!-- #post -->

            <?php endwhile; // end of the loop. ?>

        </div>
    </main>
    </section>
</div>
<?php include("footer.php"); ?>