Both phrases survive only in the works of Plutarch. This moral philosopher was active in the late 2nd century AD, some 450 years after the death of Philip II. It's often impossible to know where he got his information from, and we don't know if either of the famous replies you've cited is real - especially because they don't occur in any other source. I wrote in more detail about the 'if' response here.

In any case, the relevant bits of Plutarch are these (with text and translation from the Loeb Classical Library editions, which I've adjusted to be as literal as possible):

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Φιλίππου γράφοντος, ὅτε εἰς τὴν χώραν αὐτῶν παρεγένετο, πότερον βούλονται φίλιον ἐλθεῖν ἢ πολέμιον αὐτόν, ἀντεφώνησαν, ‘οὐδέτερον.’

When Philip wrote, at the time when he entered their country, whether they wished that he should come as a friend or enemy, they answered, "Neither."

-- Plutarch, Sayings of the Spartans 69.28 (Moralia 233e)

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καὶ πάλιν γράψαντος αὐτοῖς τοῦ Φιλίππου ‘ἂν ἐμβάλω εἰς τὴν Λακωνικήν, ἀναστάτους ὑμᾶς ποιήσω, ’ ἀντέγραψαν ‘αἴκα.’

And when Philip wrote to them: "If I invade Lakonia, I will bring about your ruin," they wrote back, "If."

-- Plutarch, On Talkativeness 17 (Moralia 511a)

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So, the words you're looking for are οὐδέτερον oudeteron, meaning literally "neither of the two," and αἴκα aika, the Doric dialect form of ean, meaning "if".