This paper develops the ontology of the photon within the Dynamic Principles of the Substrata (DPS) framework, in which the vacuum is modeled as a hyper-elastic continuum admitting three thermodynamic phases. A photon is identified as a propagating phase-transition event: a sub-stable fold that must traverse the substrate at the characteristic speed c in order to satisfy an instantaneous stability condition. From a single stability budget governing all substrate folds, the paper derives: (1) the photon velocity c as an existence condition rather than a kinematic postulate; (2) the proportionality E ∝ f from transverse compression dynamics; (3) bounded photon amplitude, predicting intensity as photon flux; (4) momentum transfer as stability-budget reallocation during absorption; and (5) argues for the relativistic energy–momentum relation E² = (pc)² + (m₀c²)² as a consequence of orthogonal energy channels within the stability budget. Five falsifiable predictions are identified, including anomalous pair-production thresholds near Schwinger-scale fields and substrate-frame anisotropy in cavity QED.