A prepositional phrase can also be an adverb phrase, functioning as an adverb, as in the following sentences.

She bought some spinach when she went to the corner store.

In this sentence, the prepositional phrase "to the corner store" acts as an adverb modifying the verb "went."

Lightning flashed brightly in the night sky.

In this sentence, the prepositional phrase "in the night sky" functions as a adverb modifying the verb "flashed."

In early October, Giselle planted twenty tulip bulbs; unfortunately, squirrels ate the bulbs and none bloomed.

In this sentence, the prepositional phrase "in early October" acts as an adverb modifying the entire sentence.

We will meet at the library at 3:30 P.M.

In this sentence, the prepositional phrase "at 3:30 P.M." acts as an adverb modifying the verb phrase "will meet."

The dogs were capering about the clown's feet.

In this sentence, the prepositional phrase "about the clown's feet" acts as an adverb modifying the verb phrase "were capering."