Check my first reply for a more detailed explanation, but the short version is that, each time the function calls itself, the caller stops and waits for the "child" function to finish. So none of the instances reach that final return until one of them returns from the first return, then they all finish and hit the second return in reverse order. Something like this:

binary1 calls binary2, stops and waits for it
  binary2 calls binary3, stops and waits for it
    binary3 calls binary4, stops and waits for it
      binary4 returns from the first return line ($n==1)
    binary3 finishes, returns
  binary2 finishes, returns
binary1 finishes, returns
print statement executes, printing binary1's return value
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