[themes] php and learning to theme

[brendan carney]

hello! 
my first post in this list..
what areas of PHP should i get familiar with (first) in order to really 
customize themes?

a little background:  i've been poking around drupal for a while, and am 
starting to get a feel for it.  i consider my knowledge of modern 
table-less xhtml/css practices to be somewhere between intermediate and 
advanced.  my knowledge of PHP (and programing in general) is very 
limited.  i've made it through a few tutorials, and parts of a rather 
poorly written php book.  in short, i know enough about php to be 
willing to experiment, but it's still a little scary. (but only a little) =)

i love xhtml/css, falling hard for drupal, and am excited to start 
learning PHP.  as i'm a graphic designer, being able really customize 
themes is of extreme interest, and an area of the community i'd like to 
be able to contribute to in the near future.

i'm planing to start going through online php tutorials more regularly 
in order to become more familiar with php in general, and specifically 
so that i can customize and develop drupal themes to my heart's content. 

so again, what areas in php should i particularly focus on with regards 
to themeing?

thanks!