Some examples of prepositional phrases functioning as adverb phrases would be the following:
Ross rode his brand new bicycle across the bridge. (“rode” = verb being described; “across” = preposition; “across the bridge” = adverb phrase)
Beatrice memorized the vocabulary words before the quiz. (“memorized” = verb being described; “before” = preposition; “before the quiz” = adverb phrase)
Our class filed out of the school building when the fire alarm rang. (“filed” = verb being described; “out” = preposition; “out of the school building” = adverb phrase)
With extreme caution, Jonathan pulled into the narrow parking space. (“pulled” = verb being described; “with” = preposition; “with extreme caution” = adverb phrase)