Purpose of !$ and difference b/w isset and empty?

Question

1. What is the purpose of !$ in PHP?
   
2. What is the difference between isset and empty?

Answer 1

1. !$ means NOT thus if (!$a) and $a == TRUE then it is NOT TRUE
   Reference: http://www.php.net/manual/en/language.operators.logical.php

2. ISSET returns TRUE if the variable exists and has a value other than NULL
EMPTY checks to see if a variable is empty.
   Reference: http://php.net/manual/en/function.isset.php http://php.net/manual/en/function.empty.php