For some people, the concern about what might happen to the mysql license is very important. I am not in that camp at present, and don't despise Oracle and everything it stands for on principle. It's going into the 4th year since Oracle bought Sun, and the sky has not fallen, nor has Oracle stopped enhancing it, or pulled it off the market, or closed sourced it. I'm sure they are much more concerned with MongoDB than they are with alternatives to MySQL. Still it's no surprise that people would be very interested in what the mysql founder is doing with his fork and embrace that. So it's no surprise that MariaDB is default in some distributions, although I don't think a relational database should be the default in a distribution in the first place.
For many people, there is no noticeable difference between the two, and have more in common in their lineage than differences. I personally never use mysql/myisam, and instead use the innodb engine when I use mysql. At this juncture there is a choice you have to make with mariadb -- do you utilize the Xtradb (innodb fork) or innodb as an engine. It's interesting looking at, but I'm not sure that there's any huge advantage to mariadb w/innodb or xtradb for that matter, but perhaps for some uses there might be.
There may in some cases be performance improvements in MariaDB and there are some interesting edge-case plugins that exist for it, none of which I see as essential.
A really good summary exists here: http://slashdot.org/...l-a-comparison/
With that said, there is probably a lot about mysql you really want to learn, before you start concerning yourself with this issue.
In conclusion, there is no real difference from the client/php perspective between the two right now.