The only secure server you will ever find is one your build yourself......and you will still have security issues. This is not to say that PHP code that is written well, on an updated server system, with a firewall is not safe....It is just that there are levels of security. This type of question really can't be answered, as it is not a direct question. I suggest you get a hold of a Linux or a BSD distro and install Apache, PHP, a database like MySQL or Postgresql and learn to configure your own server. This is not expensive anymore, as you could use a P-500 with 128 mb of RAM quite comfortably. If you are really serious about security, you should try http://www.openbsd.org
If you are knew to system admin perhaps Ubuntu Linux, Fedora Core Linux or Mandrake Linux. I am biased towards Fedora because of the ease of installing the LAMP package.