How to find and replace files contents that match a pattern?

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http://unix.stackexchange.com – Suppose we have a folder named Project. I would like find each file in the folder that contains the pattern &$ at the function call. For example should replace: AnyFunction(&$arg); with AnyFunction(&$arg); Where AnyFunction is an unknown function that matches the pattern. I need this to try to solve this php problem. How can I perform this? (HowTos)