Custom wordpress registration form

myreg.php

<?php
/*
 * Plugin Name: My Registration
 * mkdir wp-content/plugins/myreg
 */
// Add new form element
add_action('register_form', 'myreg_register_form');

function myreg_register_form() {
  if(!empty($_POST['first_name'])) {
    $first_name = trim($_POST['first_name']);
  } else {
    $first_name = '';
  }
?>
<p>
  <label for="first_name"><?php _e( 'First Name', 'mydomain' ) ?><br />
  <input type="text" name="first_name" id="first_name" class="input" value="<?php echo esc_attr( wp_unslash( $first_name ) ); ?>" size="25" /></label>
</p>
<?php
}

//2. Add validation. In this case, we make sure first_name is required.
add_filter( 'registration_errors', 'myreg_registration_errors', 10, 3 );
function myplugin_registration_errors( $errors, $sanitized_user_login, $user_email ) {

  if ( empty( $_POST['first_name'] ) || ! empty( $_POST['first_name'] ) && trim( $_POST['first_name'] ) == '' ) {
    $errors->add( 'first_name_error', __( '<strong>ERROR</strong>: You must include a first name.', 'mydomain' ) );
  }

  return $errors;
}

// 3. Finally, save our extra registration user meta.
add_action( 'user_register', 'myreg_user_register' );
function myplugin_user_register( $user_id ) {
  if ( ! empty( $_POST['first_name'] ) ) {
  update_user_meta( $user_id, 'first_name', trim( $_POST['first_name'] ) );
  }
}