Wordpress - How to check if a user is in a specific role?

If you only need this for current user current_user_can() accepts both roles and capabilities.

UPDATE: Passing a role name to current_user_can() is no longer guaranteed to work correctly (see #22624). Instead, you may wish to check user role:

$user = wp_get_current_user();
if ( in_array( 'author', (array) $user->roles ) ) {
    //The user has the "author" role
}

I was looking for a way to get a user's role using the user's id. Here is what I came up with:

function get_user_roles_by_user_id( $user_id ) {
    $user = get_userdata( $user_id );
    return empty( $user ) ? array() : $user->roles;
}

Then, an is_user_in_role() function could be implemented like so:

function is_user_in_role( $user_id, $role  ) {
    return in_array( $role, get_user_roles_by_user_id( $user_id ) );
}

You can also just create a new user object:

$user = new WP_User( $user_id );

if ( ! empty( $user->roles ) && is_array( $user->roles ) && in_array( 'Some_role', $user->roles ) ) {
    return true;
}

Not sure what version get_user_roles_by_user_id was removed in, but it's no longer an available function.