Yes, it is entirely possible to create an entire PHP website including a booking or contact form that does not require a MySQL backend. http://www.waypoint-consulting.co.uk is built in PHP and does not use a MySQL DB as it does not need one.

To your 2nd question, its also a Yes. A MySQL database can be added to an existing site, although how much change is required to the site will depends on exactly where the database will be used. For instance, if it is only required to add some new functionality that exists on a single page, then all the other pages could well be unaffected by it.

On the subject of hosting companies, much will depend on the nature of the site in question - if its a fairly small site, with a limited number of visitors, then most shared hosting providers will work. However as traffic or complexity increases, then you will want to start looking at alternatives such as a VPS, Cloud or dedicated server. I would always recommend avoiding shared hosting for eCommerce sites, as your security can be undermined by the other people sharing your host, and this is something you have no control over. If you do go down the VPS/Dedicated server route, start with monthly contracts, as it is then easier to change providers if the one you choose doesn't quite deliver what you need.

Stuart thanks a million for such a brilliant advice mate, well appreciated. Thanks a lot