I'm trying out SlickEdit and it says it can use a "tag file" for PHP. Does anyone know where I can find this tag file?
Tag files are basically an index of names in your code project that are spread across files. This can be used for jumping between files and auto-completion. Some relevant information about the concept is here on Wikipedia: Ctags.
As for creating it in SlickEdit, check the section "Building and Managing Tag Files" in the help. You should be able to use the Tools -> Tag Files dialog to add files that you want to index.
According to "SlickEdit 2008 - Languages Supported," SlickEdit already supports PHP, out of the box. When you install it, no tag file is included? It looks like it should already exist.
It supports the ability to create a tag file on PHP source code. You still need to point it in the direction of the PHP source code.
To create these tag files, go Tools | Tag Files... | Add Tag File...
Select PHP and then add the directories containing your source code.