Insert custom data to custom table on wordpress database

i need some help here

i wanna create some form for user to subscribe our article and store the data to database. i have tried to search it on google, but nothing worked when i tried that. i know that some plugin can do that but i dont know how to create the validation when user already subscribed and the form never show again.

how do i do to insert the data to my database? hope you guys understand what the problem and any help will appreciate

thank you

here is my code

the form

                    <form action="<?php echo site_url() . '/insert-data.php'; ?>" method="POST" name="form-subscribe">                         <div class="row">                             <div class="col-12 col-lg-12 col-md-12">                                 <p>Name*</p>                                 <input type="text" name="subs_name" placeholder="Full Name*" required="">                                                            </div>                         </div>                         <div class="row mt-3">                             <div class="col-12 col-lg-12 col-md-12">                                 <p>Email*</p>                                 <input type="text" name="subs_email" placeholder="Email Address*" required="">                                                           </div>                         </div>                         <div class="row mt-3">                             <div class="col-12 col-lg-12 col-md-12">                                 <p><button type="submit" class="btn btn-primary" name="submitForm">Submit</button></p>                             </div>                         </div>                     </form> 

the action file aka insert-data.php

<?php      //setting up the form     function insertuser() {        $  name   = $  _POST['subs_name'];       $  email  = $  _POST['subs_email'];          global $  wpdb;        $  table_name = $  wpdb->prefix . "subscriber";       $  wpdb->insert($  table_name, array('subs_name' => $  name, 'subs_email' => $  email) );       }  if( isset($  _POST['submitForm']) ) insertuser();  ?> 

Here is the table of wp_subscriber enter image description here