Fatal Error

I’m adding the plugin but when I try to activate it I get this error: Fatal error: Uncaught Error: Call to undefined function ctype_digit() in /hermes/bosnacweb04/bosnacweb04ac/b2570/myd.fellwoshiplodge1175c/newbuild/wp-content/plugins/wpforo/wpf-includes/functions.php:1002 Stack trace: #0 /hermes/bosnacweb04/bosnacweb04ac/b2570/myd.fellwoshiplodge1175c/newbuild/wp-content/plugins/wpforo/wpf-includes/functions.php(2322): wpforo_bigintval('') #1 /hermes/bosnacweb04/bosnacweb04ac/b2570/myd.fellwoshiplodge1175c/newbuild/wp-content/plugins/wpforo/wpf-includes/functions.php(1314): wpforo_wp_url_to_postid('wp-admin/plugin...') #2 /hermes/bosnacweb04/bosnacweb04ac/b2570/myd.fellwoshiplodge1175c/newbuild/wp-content/plugins/wpforo/wpf-includes/functions-installation.php(844): wpforo_clean_cache('option') #3 /hermes/bosnacweb04/bosnacweb04ac/b2570/myd.fellwoshiplodge1175c/newbuild/wp-content/plugins/wpforo/wpf-includes/functions-installation.php(60): wpforo_fix_installed_options() #4 /hermes/bosnacweb04/bosnacweb04ac/b2570/myd.fellwoshiplodge1175c/newbuild/wp-content/plugins/wpforo/wpf-includes/funct in /hermes/bosnacweb04/bosnacweb04ac/b2570/myd.fellwoshiplodge1175c/newbuild/wp-content/plugins/wpforo/wpf-includes/functions.php on line 1002

How do I solve this problem?