Information on how to navigate through references

Dear webmaster! I don't know wheather I'm right or wrong contacting you with my questions, nontheless I'll hope you can help me a bit. First, I would like to thank you for your site!! I was pretty lucky as I found it. It gives me a clue how to work with PHP. Alway I tried to get some information I used the text box for searching keywords, in the top of the page, but results were'nt well all the time. For example: I'd been searching for some help on __FILE__. I do know it is no function, but what is it? I found no category what might have matched to it, so I used the term "whole site" or "online documentation"; there was no good result yet. Any link I found didn't belong to the searching term. Now my question. Is it possible to move through the diffrent chapters like 'GD' or else? Or have I to use the searching function?? Thanks for time!! Yours sincerly, Sebastian.