Seeing https://trac.osgeo.org/gdal/ticket/6770, I realized that we have in-tree PHP bindings...
They appear to still build with PHP 5 on an ancient distro, but they are broken with PHP 7.
I've tried the attached patch that attempts to use the -php7 flag available in SWIG 3.0.11 but this still results in build failures (see attached log).
Failing to find anyone interested, we'll have to disable
Spatialys - Geospatial professional services
just to inform that I've acceepted 2 pull requests in master to remove the PHP and Ruby bindings, which haven't seen any activity in years, from the source tree