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<?php

// Sets the user's display name (always) to first name last name, when it's avail.
add_action ('admin_head','make_display_name_f_name_last_name');
function make_display_name_f_name_last_name(){

  $users = get_users(array('fields'=>'all'));

  foreach($users as $user){
    $user = get_userdata($user->ID);    

    $display_name = $user->first_name . " " . $user->last_name;

    if($display_name!=' ') wp_update_user( array ('ID' => $user->ID, 'display_name' => $display_name) );
      else wp_update_user( array ('ID' => $user->ID, 'display_name' => $user->display_login) );

    if($user->display_name == '')
      wp_update_user( array ('ID' => $user->ID, 'display_name' => $user->display_login) );
  }
}


?>