Description:
------------
I am unable to create a header that doesn't have a namespace. I need to create a header in the form:

<SOAP-ENV:Header>
  <SessionHeader><sessionId> ... </sessionId></SessionHeader>
</SOAP-ENV:Header>

but the API forces me to specify a namespace for the header in the SOAPHeader constructor and it doesn't accept null or empty string.

You can find the related WSDL here: http://www2.openair.com/wsdl.pl?wsdl

Reproduce code:
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$header = new SoapHeader('', 'SessionHeader', $sessVar);

Expected result:
----------------
The header <SessionHeader><sessionId> ... </sessionId></SessionHeader> is created.

Actual result:
--------------
Error: "SoapHeader::SoapHeader(): Invalid parameters. Invalid namespace" is thrown.