Run functions only in the admin area?

Question

I'd like this code to only run inside the admin area as it is resorting the items on the public side admin bar too.

  /* Reorder Admin Menu to put "Pages" at the top */
  function menu_order_filter($menu) {
  $content_menu = array('edit.php?post_type=page');
  array_splice($menu, 2, 0, $content_menu);
  return array_unique($menu);
  }
  add_filter('custom_menu_order', create_function('', 'return true;'));
  add_filter('menu_order', 'menu_order_filter');

Answer 1

There is very little overhead to assigning couple of filters on hooks that simply won't fire on front end.

In general it would be something like this:

add_action('init', 'admin_only');

function admin_only() {

    if( !is_admin() )
        return;

    // filter assignemnts and such go here
}

Also create_function() is not recommended for performance and some other reasons. It is better to use more modern Anonymous Functions, but for cases like this WordPress provides ready-made __return_true() function.

Answer 2

use the Hook admin_init and later hooks, the init-Hook comes realy ealier as only the admin. But it is important, when you use li18n-functions or AJAX, than it is better you use init.

Answer 3

https://developer.wordpress.org/reference/functions/is_admin/

if ( ! is_admin() ) {
     echo "You are viewing the theme";
} else {
     echo "You are viewing the WordPress Administration Panels";
}

Answer 4

Update in 2020, Working answer, I went to different answers none of them works now in new WordPress.

function hide_categories_for_specific_user( $exclusions, $args ){

if ( ((defined( 'REST_REQUEST' ) && REST_REQUEST) or $GLOBALS['pagenow'] === 'edit.php' ) && !current_user_can( 'manage_options' ) ) {

   // IDs of terms to be excluded
   $exclude_array = array("12","16","17"); // CHANGE THIS TO IDs OF YOUR TERMS


   // Generation of exclusion SQL code
   $exterms = wp_parse_id_list( $exclude_array );
   foreach ( $exterms as $exterm ) {
           if ( empty($exclusions) )
               $exclusions = ' AND ( t.term_id <> ' . intval($exterm) . ' ';
       else
               $exclusions .= ' AND t.term_id <> ' . intval($exterm) . ' ';
   }

   // Closing bracket
   if ( !empty($exclusions) )
   $exclusions .= ')';

   // Return our SQL statement

  }
   return $exclusions;
}

 // Finally hook up our filter
 add_filter( 'list_terms_exclusions', 'hide_categories_for_specific_user', 10, 2 );

answer final help from Hide Some Categories in Post Editor