Determine the use of the prepositional phrase in the following sentence.

we found the trinket within the old trunk.

is it an adjective or adverb?

Adjective.

Example:
In the given statement, "The Caribbean sea is a great location for deep-sea fishing." The phrase "a great location for deep-sea fishing" is the adjective phrase of the sentence.

Phrases are group of words that doesn't have a complete thought unlike clauses that can stand alone since it has a subject and predicate whereas a phrase doesn't.
An adjective phrase is one type of phrase to describe a noun or pronoun subject in a sentence. Also is adverb clause.