If You get such error on after installing cacti, and no mater how big memory_limit:

[Mon Nov 17 22:41:49.267134 2014] [:error] [pid 24152] [client 78.58.224.49:41062] PHP Fatal error:  Allowed memory size of 2147483648 bytes exhausted (tried to allocate 130968 bytes) in /usr/share/cacti/lib/adodb/adodb.inc.php on line 833

Please make sure if You imported the cacti sql file.
My .sql file location was:
/usr/share/doc/cacti-0.8.8b/cacti.sql

You may easily import the database like:

cat /usr/share/doc/cacti-0.8.8b/cacti.sql | mysql -u username database -p

More info:
http://docs.cacti.net/manual:087:1_installation.1_install_unix.5_install_and_configure_cacti