Description:
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What I'm actually trying to do is to create pointer to session.
Ok. Piece of cake.
STEP:1
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session_start();
$a = &$_SESSION;
$a[5] = 555;

 -> RUN THE SCRIPT NOW;
back to edit:

session_start();
$a = &$_SESSION;
$a[5] = 555;
print_r($_SESSION);

this echoes correctly ;) 

STEP:2
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Let's try to zero-memory session data.

session_start();
$a = &$_SESSION;
$_SESSION=NULL; 
$a[5] = 55;
$_SESSION['abc']=555;
print_r($_SESSION);

At this point $_SESSION contains two elements.
No wonder, right?  Yea..right...

Now back to edit:

session_start();
print_r($_SESSION);

At this point session contains only ONE element?
WTF happend to the $_SESSION[5]=55; it was there before... 

My first shot was to unset session data with session_unset so i repleaced 
SESSION=NULL with session_unset(). This gives me the same effect.

I also took a chance with session_destroy ;)  but then the SESSION table was empty (and that makes perfecly sense).

However its hard to understand why $_SESSION table is saved partialy and not including values setted by $_SESSION reference, and WHY THIS ONLY HAPPEND WHEN I DO SOMETHING WITH THIS TABLE to erase it ;P

Test script:
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TEST SCIRPT IS PART OF DESCRIPTION.

Expected result:
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All variables setted by SESSION reference after calling session_unset() are expected to stay in SESSION.

So:
    first i make session ref: $a = &$_SESSION;
    then i zero-memory session by session_unset();
    then i set something to session by ref.  $a[5]=55;
    and i expected this to stay in session.

Actual result:
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If i use session_unset() or nullarize session by $_SESSION=NULL and then set variables by session reference...The variables set by session reference are gone after script ends.