isset() and empty() are the function of PHP which are used to check whether the variable contains some value or not.

The major difference between them is::

isset() is used when we want to just check whether the variable is set or not. It returns TRUE even if the variables contains value like::
 

$var = NULL;
$var = FLASE;
$var = 0; 
$var = "";
$var = "0";

It will return FALSE in a case like this:
 

$var;
isset($var);  //returns false

Whereas, empty() checks whether the variables is set or not  as well as the truthness of the variable.

For example empty() will return TRUE if the variable is empty in cases like this::

$var;  //returns true as this variable is null
$var = NULL;
$var = FLASE;
$var = 0; 
$var = "";
$var = "0";