If you are simply buying some web hosting space (as opposed to a dedicated server of some kind, including a VPS), which version of PHP (if indeed any) is installed is out of your control.
If you are using a dedicated server, VPS or similar, and need to ask this kind of question, you should reconsider whether such hosting solutions are appropriate in your situation. Running an Internet-facing server is a large responsibility, not to be undertaken lightly.
Bottom line, assuming you aren't running your own server or server instance, you will need to contact your web host's technical support department about this problem. If they cannot answer the question, they will have the ability to escalate it internally to someone who can.