list_random. This function returns one of its arguments chosen at random. So
$var = list_random('w','x','y','z');would select the variable name at random, and could replace your three-line version of doing this.
[Of course, you could have done your version in only one line as well,
$var = ('w','x','y','z')[random(0,3,1)];but
list_randomis slightly cleaner, as you don't have to worry about knowing the number of elements in the array.]