Under the php_parent node, you may use the tag to specify where the file hosting the php parent class is defined. This element is missing in the 3.2/itop_design.xsd file.
<php_parent>
<name>namespace\MyPhpClass</name>
<file>file_path_in_the_extension/file.php</file>
</php_parent>
For instance, I have create, in TeemIp Cable Management, the PHP _NetworkSocket.php file to store all the methods that are related to the NetworkSocket class defined in XML:
<php_parent>
<name>TeemIp\TeemIp\Extension\CableManagement\Model\_NetworkSocket</name>
<file>src/Model/_NetworkSocket.php</file>
</php_parent>