Write a programming language interpreter [duplicate]

Possible Duplicate:

References Needed for Implementing an Interpreter in C/C++

How do I write a programming language interpreter such as how PHP works, im not talking about compilers though, are there any simple examples that can get me started writing a programming language interpreter?

Have a look at boost.spirit - its a good place to start. (in particular - the writing parsers section of the docs)

By no means a trivial amount of work if you are talking about a full featured programming language but of course doable with parsers and lexical analyzer such as flex