Add a drop down list to comment form?

Can somebody help me to create a drop-down menu or (radio buttons) in WordPress comment form, so that a new user could make a selection of their user roles (for example teachers + students) ?

1. Output from dropdown list or radio button would appear somewhere in
   the comments area.
2. The best of all would also be if WordPress which already automatically populate
   email, login name would also display new extra field info (dropdown list OR
   radio button) to logged in users who already has their user roles
   assigned
3. I don’t want to use a plugin for that.
4. I know that there a lot of tutorials about adding extra custom
   fields to the comment form including this web where I have found 3
   similar requests unfortunately marked as duplicates, on my opinion
   they are not dups at all, because there’s no code or tutorial for
   using known values as (select – dropdown- radio) button.

• Filter comment_form_field_comment to add a select element with a label.
• Add a callback to the action comment_post to save the value.
• Filter comment_text to show the value for a comment.

Sample code:

add_filter( 'comment_form_field_comment', function( $field ) {

    global $wp_roles;

    $user = wp_get_current_user();

    $select="<p><label for="roleselect">Your role:</label>
    <select name="prefix_role" id="roleselect">
    <option value="">Select a role</option>";

    foreach ( $wp_roles->roles as $key => $role )
        $select .= sprintf(
            '<option value="%1$s" %2$s>%3$s</option>',
            esc_attr( $key ),
            ( in_array( $key, $user->roles) ? 'selected' : '' ),
            esc_html( $role['name'] )
        );

    $select .= '</select></p>';

    return $select . $field;
});

add_action( 'comment_post', function( $comment_ID ) {

    $roles = new WP_Roles;
    $role_keys = array_keys( $roles->roles );

    if ( isset ( $_POST['prefix_role'] ) and in_array( $_POST['prefix_role'], $role_keys ) )
        update_comment_meta( $comment_ID, 'prefix_role', $_POST['prefix_role'] );
});

add_filter( 'comment_text', function( $text, $comment ) {

    if ( $role = get_comment_meta( $comment->comment_ID, 'prefix_role', TRUE ) )
        $text = "Role: $role<br> $text";

    return $text;
}, 10, 2 );

Update

I have rewritten the code to use a real MVC pattern. Explanation below.

Plugin header

<?php # -*- coding: utf-8 -*-
namespace WPSE;
/**
 * Plugin Name: Comment Meta Demo
 * Description: Create, save and display a comment meta field. Here, a commentator can select a role.
 * Plugin URI:  http://wordpress.stackexchange.com/q/101579/73
 * Version:     2013.06.06
 * Author:      Fuxia Scholz
 * Licence:     MIT
 * License URI: http://opensource.org/licenses/MIT
 */

\add_action(
    'wp_loaded',
    array( __NAMESPACE__ . '\Comment_Meta_Controller', 'init' )
);

Controller

/**
 * Controller
 *
 * Assigns Views and models to actions and filters
 */
class Comment_Meta_Controller
{
    /**
     * Callback for add_action(). Creates a new instance.
     *
     * @wp-hook login_init
     */
    public function init()
    {
        return new self;
    }

    /**
     * Set up objects, register footer action callback.
     *
     * @wp-hook login_init
     */
    protected function __construct()
    {

        $data   = new Comment_Meta_Builtin_Roles( '_comment_role' );
        // Use this for custom roles instead
        //$data   = new Comment_Meta_Custom_Roles( '_comment_role' );
        $input  = new Comment_Meta_Role_Selector( $data );
        $output = new Comment_Meta_Role_Display( $data );

        // remove this if you want to show the select field with
        // do_action( 'comment_role_selector' );
        \add_filter( 'comment_form_field_comment', array ( $input, 'show' ), 10, 2 );

        \add_action( 'comment_role_selector', array ( $input, 'print_select' ) );

        // remove this if you want to show the select field with
        // do_action( 'comment_role_selector' );
        \add_filter( 'comment_text', array ( $output, 'show' ), 10, 2 );

        \add_action( 'comment_role_value', array ( $output, 'show_action' ), 10, 2 );

        if ( 'POST' === $_SERVER[ 'REQUEST_METHOD' ] )
            \add_action( 'comment_post', array ( $data, 'save' ) );
    }
}

Abstract meta data base class

/**
 * Base class for handling comment meta data.
 */
abstract class Comment_Meta_Data_Model
{
    /**
     * Meta key
     *
     * @type string
     */
    protected $key;

    /**
     * Constructor
     *
     * @param string $key
     */
    public function __construct( $key )
    {
        $this->key = $key;
    }

    /**
     * Get current key
     *
     * @return string
     */
    public function get_key()
    {
        return $this->key;
    }

    /**
     * Wrapper for the native get_comment_meta()
     *
     * @param  int    $comment_ID
     * @return string
     */
    public function get_comment_meta( $comment_ID )
    {
        $meta    = \get_comment_meta( $comment_ID, $this->key, TRUE );
        $allowed = $this->get_allowed_values();

        // get real display value
        if ( isset ( $allowed[ $meta ] ) )
            return $allowed[ $meta ];

        return '';
    }

    /**
     * Save comment mate data.
     *
     * @param  int  $comment_ID
     * @return bool
     */
    public function save( $comment_ID )
    {
        $role_keys = array_keys( $this->get_allowed_values() );

        if ( ! isset ( $_POST[ $this->key ] ) )
            return;

        if ( ! in_array( $_POST[ $this->key ], $role_keys ) )
            return;

        return \update_comment_meta( $comment_ID, $this->key, $_POST[ $this->key ] );
    }

    /**
     * Get user role.
     */
    public function get_current_value()
    {
        $user = \wp_get_current_user();

        if ( empty ( $user->roles ) )
            return array ();

        return $user->roles;
    }

    /**
     * @return array
     */
    abstract public function get_allowed_values();
}

Extended class for built-in roles

/**
 * User roles as comment meta.
 */
class Comment_Meta_Builtin_Roles extends Comment_Meta_Data_Model
{
    /**
     * (non-PHPdoc)
     * @see WPSE.Comment_Meta_Data_Model::get_allowed_values()
     */
    public function get_allowed_values()
    {
        global $wp_roles;

        if ( empty ( $wp_roles ) )
            $wp_roles = new \WP_Roles;

        $output = array();

        foreach ( $wp_roles->roles as $identifier => $role )
            $output[ $identifier ] = $role['name'];

        return $output;
    }
}

Extended class for custom selection of allowed roles

/**
 * Custom roles for comment meta.
 */
class Comment_Meta_Custom_Roles extends Comment_Meta_Data_Model
{
    /**
     * (non-PHPdoc)
     * @see WPSE.Comment_Meta_Data_Model::get_allowed_values()
     */
    public function get_allowed_values()
    {
        return array (
            'teacher' => 'Teacher',
            'student' => 'Student'
        );
    }
}

Basic comment meta view

/**
 * Base class to show comment meta data.
 */
class Comment_Meta_View
{
    /**
     * Model
     *
     * @type Comment_Meta_Data_Model
     */
    protected $data;

    /**
     * Constructor.
     *
     * @param Comment_Meta_Data_Model $data
     */
    public function __construct( Comment_Meta_Data_Model $data )
    {
        $this->data = $data;
    }
}

Use a select field as view

/**
 * Show role selector from comment meta
 */
class Comment_Meta_Role_Selector extends Comment_Meta_View
{
    /**
     * Add 'select' field before textarea.
     *
     * @param  string $text_field
     * @return string
     */
    public function show( $text_field )
    {
        return $this->get_select() . $text_field;
    }

    /**
     * Select element.
     *
     * @return string
     */
    public function get_select()
    {
        $allowed = $this->data->get_allowed_values();
        $current = $this->data->get_current_value();
        $key     = $this->data->get_key();

        // is the current value part of the allowed values?
        if ( ! empty ( $current ) && array() !== array_intersect( $allowed, $current ) )
            return $this->get_hidden_field( $key, $current[0] );

        $select="<p>";
        $select .= sprintf( '<label for="%1$s_id">Your role:</label>
            <select name="%1$s" id="%1$s_id">',
            $key
        );
        $select .= '<option value="">Select a role</option>';

        foreach ( $allowed as $internal => $display )
            $select .= sprintf(
                '<option value="%1$s">%2$s</option>',
                \esc_attr( $internal ),
                \esc_html( $display )
            );

        return $select . '</select></p>';
    }

    /**
     * Print preselcted role as hidden input field.
     *
     * @param  string $name Field name
     * @param  string $role Internal role name
     * @return string
     */
    protected function get_hidden_field( $name, $role )
    {
        return sprintf(
            '<input type="hidden" name="%1$s" value="%2$s">',
            $name,
            esc_attr( $role )
        );
    }

    /**
     * Callback for do_action.
     *
     * @wp-hook comment_role_selector
     * @return  void
     */
    public function print_select()
    {
        print $this->get_select();
    }
}

Show the current role as view

/**
 * Show current comment role.
 */
class Comment_Meta_Role_Display extends Comment_Meta_View
{
    /**
     * Add role to comment text.
     *
     * @wp-hook comment_text
     * @param   string $text
     * @param   object $comment
     * @return  string
     */
    public function show( $text, $comment )
    {
        $role = $this->data->get_comment_meta( $comment->comment_ID );

        if ( '' !== $role )
            $text = "Role: $role<br> $text";

        return $text;
    }

    /**
     * Print the comment meta value into a template.
     *
     * Usage: <code>do_action( 'comment_role_value', 'Role: %s<br>', $comment );
     *
     * @wp-hook comment_role_value
     * @param   string $template
     * @param   object $comment
     * @return  void
     */
    public function show_action( $template, $comment )
    {
        $role = $this->data->get_comment_meta( $comment->comment_ID );

        if ( '' !== $role )
            printf( $template, $role );
    }
}

As you can see, we have seven classes now:

1. Comment_Meta_Controller
   Here, the other classes are combined to do something useful.
2. Comment_Meta_Data_Model
   Base class to handle the comment data. Cannot be used as is, must be extended.
3. Comment_Meta_Builtin_Roles
   Extends Comment_Meta_Data_Model and uses all built-in roles. I have used this for my tests; you should probably use the next class. Change the Controller to do that.
4. Comment_Meta_Custom_Roles
   Extends Comment_Meta_Data_Model. An alternative for Comment_Meta_Builtin_Roles.
   As you can see, you have to change just one method (function) to return an array of custom roles.
5. Comment_Meta_View
   Base class for output. Cannot be used as is, must be extended.
6. Comment_Meta_Role_Selector
   Extends Comment_Meta_View. Creates the select element. It doesn’t know anything about the source of your data and gets its values right from the View.
7. Comment_Meta_Role_Display
   Extends Comment_Meta_View. Shows the current value for a comment.

Usage

To show the select field either …

• Do nothing and let my defaults do the work. The select field will be set right above the comment text field then.

• Or remove the line …

  \add_filter( 'comment_form_field_comment', array ( $input, 'show' ), 10, 2 );
  

  … and use in your comment form this code:

  do_action( 'comment_role_selector' );
  

To set custom values for the allowed roles, remove the line …

    $data   = new Comment_Meta_Builtin_Roles( '_comment_role' );

… and uncomment the following line. Then edit Comment_Meta_Custom_Roles.

To change the meta key, just change the value '_comment_role'. Make sure, you don’t use a built-in key from WordPress.

That’s all.