$ apt-get install apt-transport-https
$ curl https://packages.sury.org/php/apt.gpg | apt-key add -
$ echo 'deb https://packages.sury.org/php/ stretch main' > /etc/apt/sources.list.d/deb.sury.org.list
$ apt-get update

$ apt-get install php5.6-cgi

$ apt-get install php5.6-mysql

The following PHP versions are available : 5.6.40 (/usr/bin/php-cgi5.6), 7.0.33 (/usr/bin/php-cgi7.0)

#deb https://packages.sury.org/php/ stretch main

[Tue Jan 22 15:08:58.713966 2019] [fcgid:warn] [pid 1023] (104)Connection reset by peer: [client MYIP:62256] mod_fcgid: error reading data from FastCGI server
[Tue Jan 22 15:08:58.714041 2019] [core:error] [pid 1023] [client MYIP:62256] End of script output before headers: test.php