PH helps enforce correct usage of the phonetic "an" vs. "a" for nouns and adjectives beginning with a vowel.
+----Ds**x---+
+-PHc+---A---+
| | |
I ate a green apple
+--------Ds**x--------+
| +-------A-------+
+-PHv-+ +---A---+
| | | |
I ate an edible green apple
+-Ds**v-+
| |
I ate an apple
The determiner "a" has the disjuncts
(Ds**c+ or (PHc+ & Ds**x+)
on it, while "an" has
(Ds**v+ or (PHv+ & Ds**x+).
The connector Ds**c+ allows "a" to link only to nouns that begin with a consonant, unless there is an intervening modifier that begins with a vowel. In that case, the disjunct PHc+ & Ds**x+ is used. The PHc+ prevents "a" from linking to any words immediately on the right that begin with a vowel. The Ds**x+ links to any noun, as it is now too distant for any phonetic modification to have an effect.
See also the "D" connector.