Can someone tell me if ALL the applications available on the desktop OS are available on the server version ?
I have yet to find an application that runs on Mac OS X (client) that doesn't run on Mac OS X Server. That said, I don't generally run my servers as workstations, so there may be some oddball app out there. For the most part, though, they're the same - at least from an application level.
Is the server version a "desktop + additional services" or some of the desktop apps are missing ?
"missing"…? as in 'bundled'? sure. But "missing" as in "are not available", not so much.
There are some under-the-hood differences between Mac OS X client and Server versions, mostly related to process limits and networking, but not something you're likely to notice on lightweight sites (e.g. personal use). I wouldn't want to run a multi-million-page-view-per-day web site on Mac OS X client, but Server handles it just fine.
As has already been mentioned, PHP and SMB are available in Mac OS X client - the same versions that are available in Mac OS X Server. The main difference here, though, is in the administration of the services themselves. Mac OS X Server comes with a set of GUI applications to configure your services. Mac OS X Client has either a preference pane (e.g. Windows File Sharing on or off) or requires manual editing of service configuration files. If you're comfortable with that then there's nothing wrong with using Mac OS X Client for your personal use. If you want/need a GUI to configure your web sites, though, then Server may be a better way to go.
If you're unsure you can always start with client and upgrade to Server later, although you'll pay more in the long run, of course.