Is there a Pennsylvania act or law that says that all real estate transactions have to be put in writing?

Answer 1

Yes. It is called the Statute of Frauds, and EVERY state has pretty much the same law.
Anything equal to or more than $500 must be in writing.
Any service that cannot be performed in less than one full year must be in writing.
It has been amended in some jurisdictions to specifically state that real estate transactions must be in writing, but that's a needless amendment. After all, how much real estate can you buy for less than $500?
You may also consult the Restatement 2d of Property, which says pretty much the same thing, and it the model upon which all state laws are based with respect to real property.

Answer 2

Most civilized states (excluding Texas and Louisiana, for example) have civil codes that require all transactions of any kind over $100 in value be in writing. Certainly any form of real estate contract would fall under that requirement, unless it's a simple easement or something like that.
As the prior answer correctly points out, the statute of frauds is applicable as well.

Answer 3

With an oral contract on real estate in ANY state, either party can later claim no contract was ever made. This is known as the Statute of Frauds.

http://en.wikipedia.org/wiki/Statute_of_frauds