"A phrase is a group of words that lacks a subject, a predicate, or both and does not express a complete thought. Therefore, a phrase can never stand on its own as a complete sentence." 
"Modifiers are words, phrases, or clauses that provide description in sentences ... Modifiers can be adjectives, adverbs, absolute phrases, infinitive phrases, participle phrases, prepositional phrases, adjective clauses, and adverb clauses." 
"In grammar, a modifier or qualifier is a word that modifies another word, a phrase, or a clause. In English, there are two kinds of modifiers: adjectives, which modify nouns and pronouns, and adverbs, which modify verbs, adjectives, and other adverbs. A modifier phrase is a phrase that acts as a modifier; English has adjective phrases and adverb phrases."
"Adverbial clauses (or particle phrases) such as "of course", "as it were", etc., commenting on the rest of the sentence or what has gone before in a previous sentence, may also be classed as modifiers, as in "Of course, he was never one to be silent" or "Unfortunately, we arrived late"." 
For another excellent page on phrases (with lots of examples), see:
To be honest, I've never heard of a "phrase modifier." Sounds like something that modifies a whole phrase. "Absolute phrases" can be used to modify entire sentences (see any of the links).
An always useful research tool is the BBC's grammar page: http://www.bbc.co.uk/
In the dark blue sidebar on the left, scroll down to "Learning English."
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