If you want to search for example a method/function name in PHP and want to look for its declaration and any occurrences of this method do this:
If its your first time running the search, setup the Working Set first by doing step 1:
CTRL + H -> Scope section -> Click Choose -> Click New -> Select PHP -> Type a working set name -> choose your desired working set to search.
The step above will store your working set to search for for future searches.
IF you dont want to bother with storing working sets. Right Click on your working set on your left panel -> Select Search
CTRL + H -> PHP Search (Tab) -> Search For: (Method name) option -> Limit to: (All occurences) option -> Then click Search button
A thing to note. If you are searching for a class name you need to change the "Search For" option to "Type" or your search will not work!