Description:
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No warning about using not allowed characters as a key in array $_SESSION. It is illogical, and should raise a warning or notice.

In manual it is written that $_SESSION is a superglobal *associative array* (http://pl.php.net/manual/pl/reserved.variables.session.php). Additional restrictions to key names are illogical and unintuitive. Why do a session serialization function uses diffrent algorithm than default serialize() php function which works fine in such situations? 

Example. Using | character as a key in $_SESSION array causes damaging a session file (it gets 0 bytes length) without any notice, warning or error. 

I understand that it is mentioned in manual, but it is illogical, unintuitive and should raise at least a warning.


Reproduce code:
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/* Here is php code which reproduces this error. Fire it *twice*, and look at output of a *second* execution. */

session_start();

echo "This was loaded from session:<br />\n";
var_dump($_SESSION);

$_SESSION["a|b"] = "asdasd";

echo "<br />\nThis is written to session:<br />\n";
var_dump($_SESSION);

Expected result:
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This was loaded from session:
array(1) { ["a|b"]=> string(6) "asdasd" } 

This is written to session:
array(1) { ["a|b"]=> string(6) "asdasd" } 

Actual result:
--------------
This was loaded from session:
array(0) { } 

This is written to session:
array(1) { ["a|b"]=> string(6) "asdasd" }