/usr/share/php/arc/serializers/ is in libarc-php 2.2.5-1ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/php/arc/serializers/ARC2_LegacyHTMLSerializer.php
/usr/share/php/arc/serializers/ARC2_LegacyJSONSerializer.php
/usr/share/php/arc/serializers/ARC2_LegacyXMLSerializer.php
/usr/share/php/arc/serializers/ARC2_MicroRDFSerializer.php
/usr/share/php/arc/serializers/ARC2_NTriplesSerializer.php
/usr/share/php/arc/serializers/ARC2_POSHRDFSerializer.php
/usr/share/php/arc/serializers/ARC2_RDFJSONSerializer.php
/usr/share/php/arc/serializers/ARC2_RDFSerializer.php
/usr/share/php/arc/serializers/ARC2_RDFXMLSerializer.php
/usr/share/php/arc/serializers/ARC2_RSS10Serializer.php
/usr/share/php/arc/serializers/ARC2_TurtleSerializer.php