WHERE EXPRESSION IN PHP RUNNER

I have a two view Tables. MV Service and AV service pointing to one table called service and the category is ether MV or AV depending on which view table was used to enter data.
I have another two view table call MVtask and AVtask and I have a look up on each to the service table in which I want to use Where expression so it shows only details of category MV or AV
I am using Where expression like "category=MV" or "Category=AV" but I have errors while runing it.
It must be a simle solution but it is blowing my head off. Can some one help me here.......