Hey DeMMo,
No, you do not need a server to learn php.
That being said. Yes you need a server to LEARN php.
There are people that learn languages from books and tapes. And they claim to speak a language. I have learned a few languages in my life, and only until I started using them, I really started to learn it. You run into the trouble which the book doesn't mention (: Hey, what the F.. In Central America the Spanish is different from in Spain!; Hm.. It should compile. Why doesn't this work with an extra quote).
So; yes you need a server. However, you can also set one up at home quite easily. I am unfortunately I 'windows person', by history. Have been working on a MS machine for too long, and feel that I have to re-learn to much to start using nix; been considering a mac though
If I am correct, the Mac has a build-in webserver. Otherwise I am sure apache will have a compilation suitable for the mac. The same for PhP and Mysql. Get those, install them, get frustrated, post questions in the getting started sections, and then.. Come back to discover the language.
I do not know your knowledge of computer programming. If you are new at it: You will be frustrated. Read the forums, especially how to ask the right question kind-a threads, try, and ask. Everybody here is genuinely trying to help. Some of us have unfortunately forgotten that really.. the first few weeks are hardest.
And use the manual, at php.net. It is a great resource, albeit a bit hard to use at first..
And don't forget: Have fun.
Jelle.