Dynamics 2011 (IFD) CRM SOAP Update not working from PHP web app

I am using PHP to connect to a Dynamics 2011 IFD and successfully authenticating and retrieving contacts to my web application.

I am however struggling to update a contact.

My SOAP request (below) is returning 'Bad Request' and unfortunately I do not have sufficient admin access to the server to enable more useful error reporting. Can anyone spot anything obvious in this XML which may be the issue?


    $request = '<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
     '.$this->crmAuth->GetSoapHeaderOnPremise("Update").'
    <s:Body>
    <Update xmlns="http://schemas.microsoft.com/xrm/2011/Contracts/Services" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
     <entity xmlns:a="http://schemas.microsoft.com/xrm/2011/Contracts">
    <a:Attributes xmlns:b="http://schemas.datacontract.org/2004/07/System.Collections.Generic">
     <a:KeyValuePairOfstringanyType>
    <b:key>contactid</b:key>
    <b:value i:type="c:guid" xmlns:c="http://schemas.microsoft.com/2003/10/Serialization/">'.$contactid.'</b:value>
     </a:KeyValuePairOfstringanyType>
     <a:KeyValuePairOfstringanyType>
    <b:key>firstname</b:key>
    <b:value i:type="c:string" xmlns:c="http://www.w3.org/2001/XMLSchema">'.$firstname.'</b:value>
     </a:KeyValuePairOfstringanyType>
     <a:KeyValuePairOfstringanyType>
    <b:key>lastname</b:key>
    <b:value i:type="c:string" xmlns:c="http://www.w3.org/2001/XMLSchema">'.$lastname.'</b:value>
     </a:KeyValuePairOfstringanyType>
    </a:Attributes>
    <a:EntityState i:nil="true" />
    <a:FormattedValues xmlns:b="http://schemas.datacontract.org/2004/07/System.Collections.Generic" />
    <a:Id>'.$contactid.'</a:Id>
    <a:LogicalName>contact</a:LogicalName>
    <a:RelatedEntities xmlns:b="http://schemas.datacontract.org/2004/07/System.Collections.Generic" />
     </entity>
    </Update>
     </s:Body>
    </s:Envelope>'


The 'getSoapHeaderOnPremise' function is returning working auth headers for the retrieve/retrieveMultiple requests. The only difference here being the Update method being specified:

    <a:Action s:mustUnderstand="1">http://schemas.microsoft.com/xrm/2011/Contracts/Services/IOrganizationService/Update</a:Action>