Difference between isset and empty?

ISSET checks the variable to see if it has been set, in other words, it checks to see if the variable is any value except NULL or not assigned a value. ISSET returns TRUE if the variable exists and has a value other than NULL. That means variables assigned a " ", 0, "0", or FALSE are set, and therefore are TRUE for ISSET.

           EMPTY checks to see if a variable is empty. Empty is interpreted as: " "  
           (an empty string),       0 (0 as an integer), 0.0 (0 as a float), "0" (0 as a string), 
           NULL,     FALSE, array() (an empty array), and "$var;" (a variable declared, but without a 
          value in a class.