Halliday & Matthiessen (2014: 362-3):
A phrase is different from a group in that, whereas a group is an expansion of a word, a phrase is a contraction of a clause. Starting at opposite ends, the two achieve roughly the same status on the rank scale, as units that lie somewhere between the rank of a clause and that of a word. In terms of the modal structure of the clause, prepositional phrases serve as Adjuncts, and in terms of the experiential structure, they serve as circumstances.