When writing PHP code, If you forget the dollar sign, what does PHP think you have specified?

In which part of a professional email should you try to be brief, but highly descriptive?

When early motion pictures played in movie theaters, they were often accompanied by live organ or piano music. which of the following are the most likely reasons that this happened? (select all that apply). the music was provided to distract audience members from the loud sounds made when filmstrips were changed. the music accompanied the movies because the movies were silent and audiences were used to hearing music during plays in theaters. the music usually was played before, and sometimes after the movie, as an alternative form of entertainment. the music viewers to interpret the dramatic action in the films.

How would you categorize the software that runs on mobile devices? break down these apps into at least three basic categories and give an example of each.