isset -> this variable handling functions determine whether 
a variable is set . It checks whether a variable is set 
even though it is empty.

empty -> as the term itself has already given a sign that 
it would related to something that's empty, this variable 
handling functions determine whether a variable is empty. 
It checks whether a variable has a value whether it's empty 
string, zero(0), or not set at all.

isset is used for checking whether variable is set or not 
and empty checks whether it is empty

actually, !isset and empty are one and same thing...

isset() will ONLY return true when the value of the variable
is not NULL (and thereby the variable is at least defined).

empty() will return true when the value of the variable is
deemed to be an "empty" value, typically this means 0, "0",
NULL, FALSE and empty string, anything else is not empty.

Some examples


FALSE == isset($foo);
TRUE == empty($foo);
$foo = NULL;
FALSE == isset($foo);
TRUE == empty($foo);
$foo = 0;
TRUE == isset($foo);
TRUE == empty($foo);
$foo = 1;
TRUE == isset($foo);
FALSE == empty($foo);

isset is used to check whether the variable is set or 
not ...
where as empty is used to check whether variable contains a 
value or not