/etc/php5/mods-available/vtkgdcm.ini is in php5-vtkgdcm 2.4.4-3+deb8u1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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; configuration for the php5-vtkgdcm module extension=vtkgdcm.so