Setting Session var to NULL

[zzz]

I have a variable set to session:

if ($_SESSION['myvar'] != NULL){
  $a = 123;
}else{
  $a = 0;
}

After doing an operation I need to set $SESSION['myvar'] to NULL. I've tried

$_SESSION['myvar'] == NULL;

and after it did not work I've tried

$_SESSION['myvar'] == '';

but I still keep getting the old value.

How do you set it to NULL?

[tank]

Assignment is one equal sign and not two:

$_SESSION['myvar'] = NULL;

[zzz]

dah... my bad.

[bradgrafelman]

Don't forget to mark this thread resolved.