can't see cacti page!help!

[robin713]

I install cacti and nagios, but cacti can't display at http://ip/cacti , it says internal server error. I check error_log of apache, but there is no useful info. How can I check it?
I put a index.php file
<?php
phpinfo()
?>
to test , it is working . but if i remove it . still can't display anything.
and my nagios is working fine.

[BSOD2600]

Possibly enable php error logging and check that file too?

[robin713]

I enable the error_log=/var/log/php_errors.log in php.ini , but I can't find it . what is the problem