[TYPO3-english] error:Class 't3lib_div' not found

[tom]

Hi,

I created a file: test.php under my domain, it looks like
typo3/,typo3conf/,test.php, in test.php, I put below code:
 <?php
 $cacheFile = t3lib_div::getFileAbsFileName('typo3temp');
debug($cacheFile);
 ?>
so when I run test.php, it shows:
Fatal error: Class 't3lib_div' not found

So what is the problem?
Thanks.

Tom