The latest unofficial php seems to be 4.3.8 for SME 6.x. Maybe you should aks at devinfo mailing list?
Under the topic "CentOS port" you'll find at devinfo:
... I don't think the horde 3/imp 4 currently in development will ever be php5 compatible.
PHP5
Horde is not compatible with PHP5 --e ven the current development CVS version won't run with PHP5.