SMEServer 7.3 is based on Centos version 4 (currently .4 I think).
As a result it has the php and MySQL version current in Centos 4.4, which are php4.3.9 and MySQL 4.1.20
There is an upcoming version of SMEServer (version 8 ) which is based on Centos 5, which will have Php 5 and MySQL 5, but the version is not yet ready for production use, it is currently still in alpha.