Description:
------------
Abstract: when handling php errors via set_error_handler() with the intention of throwing a catchable error, an E_FATAL error is triggered.

Details: This is similar to the error you might see when attempting to throw an exception from an objects destructor.  The internal pointer to the unwinding stack is lost and/or blown away.

without the error handler session_start with an invalid save_path will produce an E_WARNING

Reproduce code:
---------------
<?

ini_set('session.save_path', '/var/log'); 
set_error_handler('save_handler', E_ALL); 
try { 
    session_start(); 
} catch (Exception $e) { 
    echo $e->getMessage(); 
} 

function save_handler($errno, $errmsg) 
{ 
    throw new Exception('Error caught and thrown as exception: ' . $errmsg); 
}  


Expected result:
----------------
Error caught and thrown as exception: session_start(): open(/var/log/sess_3e97dad0fe4ce6f285e97593471f2c88, O_RDWR) failed: Permission denied (13)

Actual result:
--------------
Error caught and thrown as exception: session_start(): open(/var/log/sess_3e97dad0fe4ce6f285e97593471f2c88, O_RDWR) failed: Permission denied (13)
Fatal error: Exception thrown without a stack frame in Unknown on line 0