The assumption is that they developed a Semitic script based on acrophony, where the first sound of the Semitic name of an Egyptian glyph came to be the value of that glyph.    From Wordnik.com.  [The Volokh Conspiracy » Rehabilitating Pharaoh] Reference

Astronomy, happening at sunset, especially of star whose rising is at sunset. acrophony.    From Wordnik.com.  [xml's Blinklist.com] Reference