Advice Needed - php & Drupal Development

I need some advice on developing my programming skills. I've been using Drupal for a lot of the websites I build recently, for a number of reasons, but have had to outsource anything that required custom programming and custom module development. I know very little about php, only enough to be dangerous. I've been wanting for a while to step up these skills to a reasonable level, but I'm unsure of how to go about it. I'm assuming that developing for Drupal has its own quirks, but knowing php well is half the battle. Should I start out learning php, then move on to the Drupal side of things? If so, any good books you'd recommend? I've heard php is not too hard of a language to learn, so I'm semi-confident that i can. On a side note, I have a couple friends that suggest to not even go the php route and focus on .net. I guess they're starting to use kentico as a cms for large-scale sites. Should I go this route? I personally kinda feel that regardless of .net, php and php cms apps like Drupal will still have a market for a number of years.

Thanks for any help!