Howdy. Thats mostly because searchcode is trying to match exactly "eval($_GET" and currently has none in the index. Looking at GitHub it seems to come back with only 4 results which are exact and the rest as loose matches.
I am seeing a pattern that most people would prefer the loose match over exact to get back more results, but with the exact matches (if any) at the top.
I will take this onboard and see if I can improve things.
I would definitely expect the google pattern of doing exact match when in quotes. Throw in a regexp engine as well, because I tried using wildcards and that didn't do anything.
For example, if searching for vulnerabilities I would like to be able to do something like: