RHEL 6.4 php connection to Ubuntu 12.04 Mysql fails

RHEL 6.4 php connection to Ubuntu 12.04 Mysql fails

I'm installing Zen Cart on a Red Hat 6.4 Server. I've already installed
Apache, PHP and the php dependencies (php-mysql, etc..). My RHEL server
has two nics: one public untrusted, and one private trusted interface
connected to an internal network for db/file exchange. The web server
works from the outside. On the inside, my intent is to connect to an
Ubuntu Mysql server that already serves databases to various other
applications - so I know remote access works to the db.
However, when attempting to access the database server from the RHEL
server, it fails. I get a connection failed as if it can't find the
database or credentials are incorrect. A ping from RHEL to DB succeeds.
Using the same script from an Ubuntu web server to the DB server succeeds.
Any thoughts? Could it be a version mismatch in the php-mysql socket and
mysql server? RHEL 6.4 is using v5.3 from the RH repos, and Ubuntu has
v5.5 of mysql server from the repos.
Thanks in advance.