In Homer, Apollo is clearly identified as a different god, a plague-dealer with a silver (not golden) bow and no solar features.
The earliest certain reference to Apollo identified with Helios appears in the surviving fragments of Euripides' play Phaethon in a speech near the end , Clymene, Phaethon's mother, laments that Helios has destroyed her child, that Helios whom men rightly call Apollo (the name Apollo is here understood to mean Apollon "Destroyer").
By Hellenistic times Apollo had become closely connected with the Sun in cult. His epithet Phoebus, Phoibos "shining", drawn from Helios, was later also applied by Latin poets to the sun-god Sol.
Coin of Roman Emperor Constantine I depicting Sol Invictus/Apollo with the legend SOLI INVICTO COMITI, c. 315. The identification became a commonplace in philosophic texts and appears in the writing of Parmenides, Empedocles, Plutarch and Crates of Thebes among others, as well as appearing in some Orphic texts. Pseudo-Eratosthenes writes about Orpheus in Catasterismi, section 24: "But having gone down into Hades because of his wife and seeing what sort of things were there, he did not continue to worship Dionysus, because of whom he was famous, but he thought Helios to be the greatest of the gods, Helios whom he also addressed as Apollo. Rousing himself each night toward dawn and climbing the mountain called Pangaion, he would await the sun's rising, so that he might see it first. Therefore Dionysus, being angry with him, sent the Bassarides, as Aeschylus the tragedian says; they tore him apart and scattered the limbs." Dionysus and Asclepius are sometimes also identified with this Apollo Helios.
Classical Latin poets also used Phoebus as a byname for the sun-god, whence come common references...