This is actually a documentation problem.

The convention is to give internal functions that have optional arguments a default value and have them behave the same way when the default value is given and when the argument's not passed at all.

In PHP 5.2, you could not pass an argument to get_class and have it have the same behavior as when nothing is passed. This was fixed in PHP 5.3 by making "NULL" the default argument value.

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.

<?php
get_class(NULL);
?>
Output:
Warning: get_class() called without object from outside a class in 
D:\EasyPHP\www\index.php on line 2


Comment:
1. Then why is get_class(NULL) when called from outside the class returning a 
warning message.

2. I don't understand why is it returning class name "foo" when null is passed 
to it inside member function of a class.

3. I made a function to check if the argument passed is object of same class or 
not.
<?php
class foo {
  public static check($obj) {
    if(get_class($obj) == __CLASS__) {
      echo "YES";
    } else {
      echo "NO";
    }
  }
}

$var = new foo();
foo::check($var);   // This will print YES
foo::check($uninitialized_variable)   
/*
This will also print YES because NULL is passed to get_class() in the function 
foo::check(). And when NULL is passed to a get_class inside a member function of 
the class it returns class name.
*/
?>

I think this is not a documentation problem, there is some serious bug.
:(

There is no issue except in the documentation.

Re 1: Because that is only allowed inside a class.
Re 2: Because that is what it defaults to.
Re 3: Undefined variables default to NULL. You're also getting an E_NOTICE telling you that this might be a problem. Not a bug in PHP.

I actually thought the documentation already addressed point 1 and 2, but looking at it again it appears it doesn't. I've committed one more fix.

It may be behaving as described in the documentation, but it's seriously a big 
misfeature!  I was trying to figure out why get_class was returning the name of 
the class it was being called in and not the class of the object I was passing 
into it for the best part of a morning before I realised that the variable I was 
passing in had failed to instantiate and therefore had a NULL value.  

This function has inconsistent behaviour, and it should be updated to make the 
argument mandatory, and not assume that a NULL value means the name of the 
containing class.

Agree with Gordon, +1 for a bug