This 'missing' function is defined in lib/compatible.php. So, either something has gone wrong installing moodle and you do not have all the files, or there may be a mistake in your config.php file that causes the included libraries to be missed.
Installing and upgrading help
Problems with Installing 1.4 Dev
הדיון ננעל, כך שלא ניתן יותר להגיב אליו.