How to make and save custom form in custom plugin page?

Question

I am making a plugin with a setting page in it. In its setting page I need a user registration form with only two fields: username and password. How can I save this data to the database?

Here is what I am trying to do:

public function PLUGIN_setting_page(){
    require_once PLUGIN_PATH. 'templates/admin/plugin-settings.php';
}

plugin-settins.php:

<?php
if(isset($_POST['submit'])) {
   global $wpdb;
   $table = 'wp_myp_user';
   $data = array(
      'username' => $_POST['username'],
      'password'    => $_POST['password']
   );
   $format = array(
      '%s',
      '%s'
   );
   $success=$wpdb->insert( $table, $data, $format );
   if($success){
       echo 'data has been save' ; 
    }
} 
?>
<div class="login-page">
  <div class="form" method="post" action="">
    <form class="login-form">
      <input type="text" name="ldc_username" placeholder="username" value=""/>
      <input type="password" name="ldc_password" placeholder="password" value=""/>
      <!-- <button type="submit">login</button> -->
      <?php submit_button(); ?>
    </form>
  </div>
</div>

Answer 1

There are quite of few issues with the code. I made notes to help explain the changes. References are at the bottom, make sure to read those as well.

//isset is not the correct way. It does not check to see if the post was empty - this is the correct one ($_SERVER..)
if($_SERVER['REQUEST_METHOD'] == 'POST') {
    global $wpdb;
    //Make sure this is spelled correct.
    $table = 'wp_myp_user';

    //output will check to see if it was posted correctly.
    $output = ['status' => 1];
    //You have to sanitize the fields first - for security reasons.
    $username = sanitize_text_field($_POST["ldc_username"]);
    $password = sanitize_text_field($_POST["ldc_password"]);
    $data = array(
            //Make sure this is the same name as in the database - its spelled like this correct?
        'username' => $username,
        'password' => $password
    );
    $format = array(
        '%s',
        '%s'
    );
    //I would not put it in a variable - just send it directly.
    $wpdb->insert( $table, $data, $format );
    //Here is your post check - to console if it prints 2 - your good.
    $output['status'] = 2;
    //check reference material to learn more about wp_send_json.
    wp_send_json($output);
}
//Previously you were sending it to a function?  submit_button(); ... there is no function I can see with this. So, I would leave it out. 
?>
<div class="login-page">
   //For the action - use "#" if you want the action is happen on the same page. 
    <div class="form" method="post" action="#">
        <form class="login-form">
            <input type="text" name="ldc_username" placeholder="username" value=""/>
            <input type="password" name="ldc_password" placeholder="password" value=""/>
            <button type="submit" name="submit">login</button>
        </form>
    </div>
</div>

NOTES:

https://developer.wordpress.org/reference/functions/wp_send_json/

https://developer.wordpress.org/reference/functions/sanitize_text_field/