A small but important difference between "is_null" and "isset" is the following: "is_null" tests if an *expression* (not a *variable*) is null, while "isset" tests if a *variable* has null value or is undefined. The difference is manifested in the two following experiments: 
Experiment 1: 
<?php 
function test() { return null; } 
var_dump(is_null(test())); // displays "true" 
var_dump(isset(test())); // parse error, because "test()" is not a variable 
?> 
Experiment 2: 
<?php 
error_reporting(E_ALL); 
var_dump(isset($an_undefined_variable)); // displays "false" since "$an_undefined_variable" is not defined 
var_dump(is_null($an_undefined_variable)); // displays "true" (as expected), but throws a notice because "$an_undefined_variable" is not defined. 
?>