How to Manage WordPress User Roles & Capabilities with Code | http://www.ibenic.com/how-to-manage-wordpress-user-roles-capabilities-with-code

add_role.php

<?php

add_role( 
  'new_user_role', 
  __( 'New User Role', 'yourtextdomain' ), 
  array(
    'read' => true,
    'edit_posts' => true,
    // Various Capabilities
));

check_capability.php

<?php

/**
* Checking Capability
*/

// Useful when you are not doing something for the current user, but for others 
// Example: a CRON JOB
$user = 1; // It can be an ID or WP_User Object
$capability = 'edit_posts';
if( user_can( $user, $capability ) ) {
  // Do Something
}

// Useful when you are checking something for the current user
if( current_user_can( 'edit_posts' ) } {
  // Do Something
}

// Useful when working on a multisite
if( current_user_can_for_blog( $blog_id, 'edit_posts' ) ) {
  // Do Something
}

// Useful when checking the capability for the author of the provided post
$post = 49; //It can also be a WP_Post object
if( author_can( $post, 'edit_posts' ) ) {
  // Do Something
}

hook_add_role.php

<?php

add_action( 'add_user_role', 'hook_user_add_role', 20, 2 );

/**
* A user received a new role
* @param number $user_id
* @param string $role
**/
function hook_user_add_role( $user_id, $role ) {
  if( $role == 'administrator' ) {
    // This user has been added to administrator role, do something
  }
}

hook_remove_role.php

<?php


add_action( 'remove_user_role', 'hook_user_remove_role', 20, 2 );

/**
* A role removed from the user
* @param number $user_id
* @param string $role
**/
function hook_user_remove_role( $user_id, $role ) {
  if( $role == 'administrator' ) {
    // This user is not an administrator anymore, do something
  }
}

hook_set_role.php

<?php

add_action( 'set_user_role', 'hook_user_set_role', 20, 3 );

/**
* A role has been set, others were removed
* @param number $user_id
* @param string $role
* @param array  $old_roles
**/
function hook_user_set_role( $user_id, $role, $old_roles ) {
  if( $role == 'administrator' && in_array( 'subscriber', $old_roles ) ) {
    // This user is now an administrator and it was a subscriber before
    // let's do something
  }
}

remove_role.php

<?php

/**
* Removing the role
*/

remove_role( 'administrator' );

update_role.php

<?php

/**
* Updating the role
*/

$administrator_role = get_role( 'administrator' );

// Adding a new capability to role
$administrator_role->add_cap( 'custom_capability' );

// Remove a capability from role
$administrator_role->remove_cap( 'custom_capability' );

user_caps.php

<?php

/**
* Capabilities on User
*/

$user_id = 1;

// Getting the WP_User object
$user = get_userdata( $user_id );

// The user exists
if( $user && $user->exists() ) {
  
  // Add Capability
  $user->add_cap( 'edit_posts' );
  
  // Remove Capability
  $user->remove_cap( 'edit_posts' );
  
  // Remove All Capabilities from the User 
  // This will reset the capabilties to the User Role
  $user->remove_all_caps();
  
}

user_roles.php

<?php

/**
* Manage Roles for a User
*/

$user_id = 1;

// Getting the WP_User object
$user = get_userdata( $user_id );

// The user exists
if( $user && $user->exists() ) {
  
  // Remove all the previous roles from the user and add this one
  // This will also reset all the caps and set them for the new role
  $user->set_role( 'administrator' );
  
  // Remove the Role from the user
  $user->remove_role( 'administrator' );
  
  // Add a new role to the user, while retaining the previous ones
  $user->add_role( 'administrator' );
  
}

wp_role.php

<?php

$administrator_role = get_role( 'administrator' );
/* $administrator role will be: */
WP_Role => array(
  'name' => 'administrator',
  'capabilities' => array(
    'switch_themes' => true,
    'edit_themes' =>  true,
    'activate_plugins' => true,
    // Much more
  )
)

wp_roles.php

<?php

WP_Roles => array(
  'roles' => array(
    'administrator' => array(
      'name' => 'administrator',
      'capabilities' => array(
        'switch_themes' => true,
        'edit_themes' =>  true,
        'activate_plugins' => true,
        // Much more
      ),
      // Other roles
    ),
   'role_names' => array(
    'administrator' => 'Administrator',
     // Other role names
   )
   // ...
 );

These have been really helpful to me! Thank you!

One quick edit: I believe line 16 of check_capaility.php needs a ) where you have a }.