Hello Fabrice,
for building the MaxDB PHP extension you do not need the source code of SQLDBC. The newest headerfiles and libraries are enought for builing and they are copied during the MaxDB installtion as SQLDBC Client installation to <installtion_path>/ sdk/sqldbc.
But you can stay at your 7.6 compilation since MaxDB 7.8 supports older clients.
Kind regards,
Burkhard