That's no problem in many modern lexers as they usually have a "state" so when you encounter "echo" you can switch to a new state and that state may have different token parsing rules. So "if" in the "echo" state could be a string literal whereas it may be a keyword in the initial state.

Lex/Flex takes care of that mostly for you which is one of the benefits of using a well worn lexer generator and not rolling your own.