This script adds capabilities to wordpress role by role name

add-user-caps.php

<?php
/* --------------------------------------------------
/ If used within a plugin, you will want to run this function on plugin activation
/ For development, you can simply call the function below . To do this, uncomment
/ the line below, then visit any page.
-------------------------------------------------- */

// custom_user_roles();

function custom_user_roles() {
  global $wp_roles;
  
  // More info:
  // http://codex.wordpress.org/Roles_and_Capabilities
  
  // Remove stock roles:
  remove_role( 'author' );
  remove_role( 'contributor' );
  
  // Add custom roles
  add_role( 'student', 'Student' );
  add_role( 'instructor', 'Instructor' );
  
  // New permissions added based on role.
  $add_new_roles = array(
    'administrator' => array(
      'publish_events',
      'read_events',
      'edit_events',
      'edit_others_events',
    ),
    'editor' => array(
      'publish_events',
      'read_events',
      'edit_events',
      'edit_others_events',
    ),
    'student' => array(
      'read',
      'read_event',
    ),
    'instructor' => array(
      'read',
      'read_event',
      'edit_event',
    ),
  );
  
  // Loop through our array, assigning the new roles.
  // If you would like to remove roles, you can replicate 
  // this code and use remove_cap instead of add_cap
  foreach($add_new_roles as $role_name => $new_roles) {
    foreach($new_roles as $role) {
      $wp_roles->add_cap( $role_name, $role );
    }
  }
}