Am I a human? Yes. Can I type phi off the top of my head? No.
According to reCAPTCHA:
Each new word that cannot be read correctly by OCR is given to a user in conjunction with another word for which the answer is already known. The user is then asked to read both words. If they solve the one for which the answer is known, the system assumes their answer is correct for the new one.
So therefore if nobody else figured out this one, you would be the first and you could legitimately type anything and succeed at it.