Getting a List of Currently Available Roles on a WordPress Site?

I'm not sure if the accepted answer is the best solution to the question, as according to the documentation, get_editable_roles() retrieves a filtered list of user roles that the current user is allowed to edit.

Maybe we just need to use wp_roles(). It retrieves the global WP_Roles instance and instantiates it if necessary. And if you just want an array with the roles ids as keys and names as values you can do this:

$roles = wp_roles()->get_names();

This is how to get an array of all the existing user roles, and the capabilities for each role, in WordPress. If you don’t want to print it to the screen, omit the last line. The $roles variable on line 2 will hold the array of users and capabilities so that you can use it however you need to. See below for an example of the returned array.

global $wp_roles;
$roles = $wp_roles->roles; 

// print it to the screen
echo '<pre>' . print_r( $roles, true ) . '</pre>';

Roles are stored in the global variable $wp_roles.

The ideal function is get_editable_roles() from /wp-admin/includes/user.php

function get_editable_roles() {
    global $wp_roles;

    $all_roles = $wp_roles->roles;
    $editable_roles = apply_filters('editable_roles', $all_roles);

    return $editable_roles;
}

The "editable" part is because it offers other plugins a chance to filter the list in case someone other than admin has 'edit_users' privilege (and thus 'admin' needs to be removed from the list, else that user could make themselves admin). Role management plugins used to create custom roles are the ones that would be using that filter. Otherwise this function is essentially get_roles() (which doesn't exist).

Presumably your plugin will only offer the settings page in question to someone who has admin-level capabilities like 'manage_options' and is basically an admin with access to all roles, so the filter shouldn't affect you.

There is also wp_dropdown_roles() which gives you the roles as <option> fields for a <select> list (though checkboxes are likely to work better in many scenarios where you're choosing who has access to something).

For those which have multilingual site, function

function wp_roles_array() {
    $editable_roles = get_editable_roles();
    foreach ($editable_roles as $role => $details) {
        $sub['role'] = esc_attr($role);
        $sub['name'] = translate_user_role($details['name']);
        $roles[] = $sub;
    }
    return $roles;
}

returns localized array like this (role names are in Slovak language):

Array
(
    [0] => Array
        (
            [role] => administrator
            [name] => Administrátor
        )

    [1] => Array
        (
            [role] => editor
            [name] => Editor
        )

    [2] => Array
        (
            [role] => author
            [name] => Autor
        )

    [3] => Array
        (
            [role] => contributor
            [name] => Prispievateľ
        )
)

Here is how you can find the list of roles without any plugins or function http://screencast.com/t/uaWsGLAR3Sh

Try this:

function get_role_names() {

global $wp_roles;

if ( ! isset( $wp_roles ) )
    $wp_roles = new WP_Roles();

return $wp_roles->get_names();
}

PS heh, missed that explanation and reply, too fast me :)