Description:
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Elements in a XML schema (XSD) may have a substitutionGroup attribute which indicates that this element may be used instead of an other element. When an element with a substitutionGroup is defined in the schema part of the WSDL and the endpoint returns a substitution instead of the original element, the SoapClient does not deserialize the XML and returns nothing. 

Reproduce code:
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Part of WSDL:
<xsd:element name="originalElement" type="xsd:string" />
<xsd:element name="substitutionElement" substitutionGroup="tns:originalElement" type="xsd:string" />

MySoapMethod returns a originalElement.

Returned XML:
<?xml version='1.0' encoding='utf-8'?>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">
<soapenv:Body>
<!-- Instead of originalElement, we return substitutionElement. It should make no difference. -->
<substitutionElement>bla</substitutionElement>
</soapenv:Body>
</soapenv:Envelope>

In PHP, call method and expect "bla" to be returned:
echo $client->MySoapMethod();

Expected result:
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Because in the example substitutionElement can be used instead of originalElement, I expect the contents of substitutionElement to be returned if there is no originalElement.

Actual result:
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An empty string is returned, if there is no originalElement.