Compile PHP 5.6, PHP 7.0, PHP 7.1, PHP 7.2, PHP 7.3 and PHP 7.4

For Debian you can use binary packages. Just follow this article:

sudo apt update 
sudo apt upgrade

apt-get install -y apt-transport-https lsb-release ca-certificates
wget -O /etc/apt/trusted.gpg.d/php.gpg https://packages.sury.org/php/apt.gpg
echo "deb https://packages.sury.org/php/ $(lsb_release -sc) main" > /etc/apt/sources.list.d/php.list

sudo apt update
sudo apt install php5.6
sudo apt install php5.6-dev php5.6-cli php5.6-fpm php5.6-common php5.6-curl php5.6-mbstring php5.6-mysql php5.6-xml

sudo apt update
sudo apt install php7.3
sudo apt install php7.3-dev php7.3-cli php7.3-fpm php7.3-common php7.3-curl php7.3-mbstring php7.3-mysql php7.3-xml php7.3-soap php7.3-xml php7.3-gd php7.3-zip php7.3-mbstring php7.3-xml php7.3-dom php7.3-bcmath php7.3-bz2 php7.3-curl php7.3-gd php7.3-intl php7.3-json php7.3-mbstring php7.3-readline php7.3-xml php7.3-zip

sudo apt update
sudo apt install php7.4
sudo apt install php7.4 php7.4-cli php7.4-cgi php7.4-fpm php7.4-gd php7.4-mysql php7.4-imap php7.4-curl php7.4-intl php7.4-pspell php7.4-sqlite3 php7.4-tidy php7.4-xmlrpc php7.4-xsl php7.4-zip php7.4-mbstring php7.4-soap php7.4-opcache libonig5 php7.4-common php7.4-json php7.4-readline php7.4-xml

If you don't want to use binary packages and instead compile from source then follow below steps:

./configure --prefix=/opt/php-7.1 --with-pdo-pgsql --with-zlib-dir --with-freetype-dir --enable-mbstring --with-libxml-dir=/usr --enable-soap --enable-calendar --with-curl --with-mcrypt --with-zlib --with-gd --with-pgsql --disable-rpath --enable-inline-optimization --with-bz2 --with-zlib --enable-sockets --enable-sysvsem --enable-sysvshm --enable-pcntl --enable-mbregex --enable-exif --enable-bcmath --with-mhash --enable-zip --with-pcre-regex --with-pdo-mysql --with-mysqli --with-mysql-sock=/var/run/mysqld/mysqld.sock --with-jpeg-dir=/usr --with-png-dir=/usr --enable-gd-native-ttf --with-openssl --with-fpm-user=www-data --with-fpm-group=www-data --with-libdir=/lib/x86_64-linux-gnu --enable-ftp --with-imap --with-imap-ssl --with-kerberos --with-gettext --with-xmlrpc --with-xsl --enable-opcache --enable-fpm
make
make install

cp /usr/local/src/php7.1-build/php-7.1.25/php.ini-production /opt/php-7.1/lib/php.ini
cp /opt/php-7.1/etc/php-fpm.conf.default /opt/php-7.1/etc/php-fpm.conf
cp /opt/php-7.1/etc/php-fpm.d/www.conf.default /opt/php-7.1/etc/php-fpm.d/www.conf
vi /opt/php-7.1/etc/php-fpm.conf

Configure PHP 7.2

mkdir -p /opt/php-7.2

mkdir /usr/local/src/php7.2-build

cd /usr/local/src/php7.2-build
wget http://de2.php.net/get/php-7.2.13.tar.bz2/from/this/mirror -O php-7.2.13.tar.bz2
tar jxf php-7.2.13.tar.bz2
cd php-7.2.13

./configure --prefix=/opt/php-7.2 --with-pdo-pgsql --with-zlib-dir --with-freetype-dir --enable-mbstring --with-libxml-dir=/usr --enable-soap --enable-calendar --with-curl --with-zlib --with-gd --with-pgsql --disable-rpath --enable-inline-optimization --with-bz2 --with-zlib --enable-sockets --enable-sysvsem --enable-sysvshm --enable-pcntl --enable-mbregex --enable-exif --enable-bcmath --with-mhash --enable-zip --with-pcre-regex --with-pdo-mysql --with-mysqli --with-mysql-sock=/var/run/mysqld/mysqld.sock --with-jpeg-dir=/usr --with-png-dir=/usr --with-openssl --with-fpm-user=www-data --with-fpm-group=www-data --with-libdir=/lib/x86_64-linux-gnu --enable-ftp --with-imap --with-imap-ssl --with-kerberos --with-gettext --with-xmlrpc --with-xsl --enable-opcache --enable-fpm
make
make install

cp /usr/local/src/php7.2-build/php-7.2.13/php.ini-production /opt/php-7.2/lib/php.ini
cp /opt/php-7.2/etc/php-fpm.conf.default /opt/php-7.2/etc/php-fpm.conf
cp /opt/php-7.2/etc/php-fpm.d/www.conf.default /opt/php-7.2/etc/php-fpm.d/www.conf

vi /opt/php-7.2/etc/php-fpm.conf
[...]
pid = run/php-fpm.pid
[...]

vi /opt/php-7.2/etc/php-fpm.d/www.conf
[...]
listen = 127.0.0.1:8998
[...]

vi /lib/systemd/system/php-7.2-fpm.service

[Unit]
Description=The PHP 7.2 FastCGI Process Manager
After=network.target

[Service]
Type=simple
PIDFile=/opt/php-7.2/var/run/php-fpm.pid
ExecStart=/opt/php-7.2/sbin/php-fpm --nodaemonize --fpm-config /opt/php-7.2/etc/php-fpm.conf
ExecReload=/bin/kill -USR2 $MAINPID

[Install]
WantedBy=multi-user.target

systemctl enable php-7.2-fpm.service
systemctl daemon-reload
systemctl start php-7.2-fpm.service
vi /opt/php-7.2/lib/php.ini
[...]
zend_extension=opcache.so

# ispconfig:
FastCGI Settings tab:
Path to the PHP FastCGI binary: /opt/php-7.2/bin/php-cgi
Path to the php.ini directory: /opt/php-7.2/lib

PHP-FPM Settings tab:
Path to the PHP-FPM init script: php-7.2-fpm
Path to the php.ini directory: /opt/php-7.2/lib
Path to the PHP-FPM pool directory: /opt/php-7.2/etc/php-fpm.d

Configure PHP 7.3

mkdir -p /opt/php-7.3

mkdir /usr/local/src/php7.3-build

cd /usr/local/src/php7.3-build
wget http://de2.php.net/get/php-7.3.0.tar.bz2/from/this/mirror -O php-7.3.0.tar.bz2
tar jxf php-7.3.0.tar.bz2

cd php-7.3.0
apt-get install libzip-dev
./configure --prefix=/opt/php-7.3 --with-pdo-pgsql --with-zlib-dir --with-freetype-dir --enable-mbstring --with-libxml-dir=/usr --enable-soap --enable-calendar --with-curl --with-zlib --with-gd --with-pgsql --disable-rpath --enable-inline-optimization --with-bz2 --with-zlib --enable-sockets --enable-sysvsem --enable-sysvshm --enable-pcntl --enable-mbregex --enable-exif --enable-bcmath --with-mhash --enable-zip --with-pcre-regex --with-pdo-mysql --with-mysqli --with-mysql-sock=/var/run/mysqld/mysqld.sock --with-jpeg-dir=/usr --with-png-dir=/usr --with-openssl --with-fpm-user=www-data --with-fpm-group=www-data --with-libdir=/lib/x86_64-linux-gnu --enable-ftp --with-imap --with-imap-ssl --with-kerberos --with-gettext --with-xmlrpc --with-xsl --enable-opcache --enable-fpm
make
make install

cp /usr/local/src/php7.3-build/php-7.3.0/php.ini-production /opt/php-7.3/lib/php.ini
cp /opt/php-7.3/etc/php-fpm.conf.default /opt/php-7.3/etc/php-fpm.conf
cp /opt/php-7.3/etc/php-fpm.d/www.conf.default /opt/php-7.3/etc/php-fpm.d/www.conf

vi /opt/php-7.3/etc/php-fpm.conf
[...]
pid = run/php-fpm.pid
[...]

vi /opt/php-7.3/etc/php-fpm.d/www.conf
[...]
listen = 127.0.0.1:8997
[...]

vi /lib/systemd/system/php-7.3-fpm.service

[Unit]
Description=The PHP 7.3 FastCGI Process Manager
After=network.target

[Service]
Type=simple
PIDFile=/opt/php-7.3/var/run/php-fpm.pid
ExecStart=/opt/php-7.3/sbin/php-fpm --nodaemonize --fpm-config /opt/php-7.3/etc/php-fpm.conf
ExecReload=/bin/kill -USR2 $MAINPID

[Install]
WantedBy=multi-user.target

systemctl enable php-7.3-fpm.service
systemctl daemon-reload
systemctl start php-7.3-fpm.service
vi /opt/php-7.3/lib/php.ini
[...]
zend_extension=opcache.so

# ispconfig:
FastCGI Settings tab:
Path to the PHP FastCGI binary: /opt/php-7.3/bin/php-cgi
Path to the php.ini directory: /opt/php-7.3/lib

PHP-FPM Settings tab:
Path to the PHP-FPM init script: php-7.3-fpm
Path to the php.ini directory: /opt/php-7.3/lib
Path to the PHP-FPM pool directory: /opt/php-7.3/etc/php-fpm.d