I have figured out how to validate an XML file using an XSD schema in PHP using this code, but it appears the processing is done using XSD version 1.0.

$dom = new DOMDocument();
$dom->loadXML($xmlString);

$isValid = $dom->schemaValidate($xsdFile);

For my case, I need the XSD file to be treated as version 1.1. The main reason for this is that I want to use:

<xs:all>
   <xs:element type="xs:string" name="br" minOccurs="0" maxOccurs="unbounded"/>
   <xs:element type="linkType" name="link" minOccurs="0" maxOccurs="unbounded"/>
</xs:all>

Notice the maxOccurs="unbounded" which can only be used inside <xs:all> with version 1.1. In version 1.0, the maxOccurs can only be 0 or 1.

PHP fails with the generic error:

Warning: DOMDocument::schemaValidate(): Invalid Schema