cf. epsilon 3140, pi 1631.
The headword phrase is a quotation from I Corinthians 15.32 (where the apostle in turn quotes Isaiah 22.13).
The aorist subjunctive regularly has a long thematic vowel. Short-voweled middle forms of φαγεῖν occur in later Greek, apparently with a future sense, e.g. Luke 14.15. These forms may be influenced by πίομαι , which serves as the future of πίνω (see LSJ s.v.).
[1] The lexicographer calls these forms "passive," but they are transitive middle forms with a sense indistinguishable from the active.