The error showed up in the httpd_log when attempting to access the /cacti site via a web browser.
The web browser would sit and eventually time-out to a blank page.
It looked like I was supposed to work with the PHP memory limits.
Fiddling with them resolved nothing.
PHP Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 261900 bytes) in ../adodb.inc.php on line 833
Turns out it was a really simple problem- I had forgotten to merge the cacti.sql from the installation to the newly created cacti dB in mySQL.
mysql --user=XXXX --password=XXXX cacti < cacti.sql
resolved my issue and the installer started to roll.