PHP tries to discover the mysql default socket path during configure
phase by probing the file system:
cf3b852109/ext/mysqli/config.m4 (L4)
This obviously fails to discover /run/mysqld/mysqld.sock, which is being
used (hardcoded) across all MySQL flavours.
This leads to PHP having no mysql socket path set for the mysql[i]
extensions, and `/tmp/mysql.sock` set for pdo_mysql,
meaning one currently has to manually configure and set it in php.ini.
Luckily, PHP supports setting that path via
`--with-mysql-sock=/run/mysqld/mysqld.sock` during configure phase,
so let's do this as soon as one of the three modules is enabled.
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