WordPress: modify user roles dynamically with functions.php

functions.php

<?php 
function custom_user_role() {
    // get user
    $user = new WP_User( <user-ID> );
    //$user = new WP_User( '<user-login-name>' );
    //$user = wp_get_current_user();

    // modify roles 
    // for example, set/unset them as administrator
    $user->add_role( 'administrator' );
    //$user->remove_role( 'administrator' );
}
// register action
add_action( 'admin_init', 'custom_user_role' );