4.3     Working with variables: isset(), empty(), and unset()

NOTE: This is NOT the latest copy of this book; click here for the latest version.

There are three very basic functions that help you use variables, and these are isset(), empty(), and unset(). Isset() and empty() are almost opposites, but not quite - isset() will return true if a variable, passed in as its only parameter, has been set in the script already, whereas empty(), which also takes a variable as its sole parameter, will return true if the variable has a false value. This is not the same thing!

To illustrate the difference consider the variable $foo being set to false - isset() would return true because $foo is a variable that has a value, and empty() would also return true because $foo's value is false. To check for "variable not set", use the not operator !, as in "if (!isset($foo)".

Unset() removes an existing variable entirely, so that isset() will return false. That variable can be recreated later on in the script, however. Use unset() when you want to delete a variable.