Jamieson, Fausset and Brown on Acts 18:23

23. And after he had spent some time there--but probably not long.

he departed--little thinking, probably, he was never more to return to Antioch.

went over all ... Galatia and Phrygia in order--visiting the several churches in succession. See on Ac 16:6. Galatia is mentioned first here, as he would come to it first from Antioch. It was on this visitation that he ordained the weekly collection (1Co 16:1, 2), which has been since adopted generally, and converted into a public usage throughout Christendom. Timotheus and Erastus, Gaius and Aristarchus, appear to have accompanied him on this journey (Ac 19:22, 29; 2Co 1:1), and from Second Corinthians we may presume, Titus also. The details of this visit, as of the former (Ac 16:6), are not given.

Ac 18:24-28. Episode Concerning Apollos at Ephesus and in Achaia.

This is one of the most interesting and suggestive incidental narratives in this precious history.