Cannot run installer, rcube_utils not found

[brassbounder]

I have recently upgraded my Mac to El Capitan, and now I cannot install Roundcubemail. It has worked perfectly with previous versions of OS X.

The installer is saying: "Fatal error: Class 'rcube_utils' not found in /usr/local/var/www/htdocs/roundcubemail-1.2-beta/program/include/bc.php on line 31."

I am running Apache 2.4.16 and PHP 5.5.29 and I have installed the full Roundcubemail package. (The beta version currently installed is only the latest, I have tried both installing and restoring from backup the current stable and the previous version with exactly the same result.)

I suspect the problem lies with PHP rather than Roundcubemail, but I was wondering if anyone could point me in the right direction, please.

Jon