Several functions do not fall under the other categories and so are covered here. The first one is
isset(), which takes one or more variables as its parameters and returns
true if they have been set. It is important to note that a variable with a value set to something that would be evaluated to
0 or an empty string—still returns
isset() because it does not check the value of the variable. It merely checks that it is set; hence, the name.
unset() function also takes one or more variables as its parameters, simply deleting the variable and freeing up the memory. With these two, you can write a script that checks for the existence of a variable and, if it exists, deletes it (see Listing 39.5).