First avoid using "strongly typed " most people misunderstood this term. (I personally refuse to give it a meaning)
I am assuming that your are talking of a statically type language as opposed to a dynamically type language.
I can understand from your background with php that you want to lean a statically type language but be aware that php is the worst dynamically type language that I have ever seen.
I would say that if you know C#, Java that is enough.
IMHO learning a language for a career POV is not what will improve your skill in PL.
This because most industries have no idea which language to choose in other to express the best there problems. So there are choosing language base on what the other competitors used. [1]
Your also have to ask your self why you want to learn statically type language.
Understand what is the difference between static and dynamic typing and what it implies. This is a hard question to answer. Is not as obvious as what most people think.
So I can answer which language while improve your programming skill ( LISP (DT) , Smalltalk (DT) , Scheme (DT) , ML (ST) , Haskell (ST), Prolog (DT), C (ST), Self (DT) ).
DT: Dynamically type,
ST: Statically type
[1] http://www.paulgraham.com/avg.html