If you want to learn more about programming for GTK, your best bet is Gnome 2.0 (Warkus), which teaches GUI programming for Gnome using GTK. Note that PHP does only support GTK+ v1, not v2, so some of the information presented in the book will not be applicable.
If you'd like to pursue the topic of games programming, you should look up Premier Press - they have an excellent Games Programming series that just keeps on growing, and each of the books are standalone in that they start from the very beginning. Particularly of interest will be the book Focus on SDL (Pazera), as all of the information presented there can be ported across to use PHP with little fuss.
There are many books available on how to write your own compiler, but only a few of these are particularly worth buying. There are, of course, no books available on writing your own compiler using PHP, but what you should really be interested in is the theory. First up, "Compilers: Principles, Techniques, and Tools", often known simply as The Dragon Book due to is front cover, (Aho, Sethi, and Ullman0 is an excellent if somewhat date book that has been the reference guide for many learning compiler writers for years now. This has been replaced to a large extent by Advance Compiler Design and Implementation (Muchnick), which has more up-to-date content. In my opinion, The Dragon Book still wins out over Muchnick's work, simply because it is presented in a more easily understood manner. Having said that, Muchnick's book does have much, much better detail on optimisation.
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