If a law in forcer obtained evidence illegally, how valid is that evidence in the court of Law?

The evidence is invalid, and cannot be admitted in court. 

Well!! Noha I believe you are more keen to know if on the grounds of illegality the case moves in favour of the wrong doer .

You can challenge the ground of illegality, move to higher courts,grounds of illegality based on the provision and if possible if at all the number of people involved in making it a false evidence.

Now depends which subject does you case falls into.

 Well I think that a court looks to an evidence in accordance with a local evidence law which means that  judge take place in processing facts and circumstances by making a balance between presented evidence by parties, thus he choose a stronger, more reliable and more stability ones ,so an evidence become legal when it matches a terms and provisions provided by evidence act.