If you want to create an soap header wihtout namespace and without an item key value setup, you can use SoapVar
To get this:
<SOAP-ENV:Header>
<IdentityHeader>
<SessionID>123456789</SessionID>
</IdentityHeader>
</SOAP-ENV:Header>
you can use this php code:
<?php
$headerVar = new SoapVar('<IdentityHeader><SessionID>123456789</SessionID></IdentityHeader>',
XSD_ANYXML);
$header = new SoapHeader('http://tempuri.org/','RequestParams',
$headerVar);
?>