In this article I will explain how to install additional versions of PHP on Debian.

You have to choices. You can install pre-compiled binary packages. Or you can compile from source.

Option 1: Install pre-compiled binaries:

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

Install PHP 5.6 binaries

sudo apt install libonig5 php5.6 php5.6-mcrypt php5.6-bcmath php5.6-bz2 php5.6-cgi php5.6-cli php5.6-common php5.6-curl php5.6-dev php5.6-fpm php5.6-gd php5.6-imap php5.6-intl php5.6-json php5.6-mbstring php5.6-mysql php5.6-opcache php5.6-pspell php5.6-readline php5.6-soap php5.6-sqlite3 php5.6-tidy php5.6-xml php5.6-xmlrpc php5.6-xsl php5.6-zip

Install PHP 7.0 binaries

sudo apt install libonig5 php7.0 php7.0-mcrypt php7.0-bcmath php7.0-bz2 php7.0-cgi php7.0-cli php7.0-common php7.0-curl php7.0-dev php7.0-fpm php7.0-gd php7.0-imap php7.0-intl php7.0-json php7.0-mbstring php7.0-mysql php7.0-opcache php7.0-pspell php7.0-readline php7.0-soap php7.0-sqlite3 php7.0-tidy php7.0-xml php7.0-xmlrpc php7.0-xsl php7.0-zip

Install PHP 7.1 binaries

sudo apt install libonig5 php7.1 php7.1-mcrypt php7.1-bcmath php7.1-bz2 php7.1-cgi php7.1-cli php7.1-common php7.1-curl php7.1-dev php7.1-fpm php7.1-gd php7.1-imap php7.1-intl php7.1-json php7.1-mbstring php7.1-mysql php7.1-opcache php7.1-pspell php7.1-readline php7.1-soap php7.1-sqlite3 php7.1-tidy php7.1-xml php7.1-xmlrpc php7.1-xsl php7.1-zip

Install PHP 7.2 binaries

sudo apt install libonig5 php7.2 php7.2-mcrypt php7.2-bcmath php7.2-bz2 php7.2-cgi php7.2-cli php7.2-common php7.2-curl php7.2-dev php7.2-fpm php7.2-gd php7.2-imap php7.2-intl php7.2-json php7.2-mbstring php7.2-mysql php7.2-opcache php7.2-pspell php7.2-readline php7.2-soap php7.2-sqlite3 php7.2-tidy php7.2-xml php7.2-xmlrpc php7.2-xsl php7.2-zip

Install PHP 7.3 binaries

sudo apt install libonig5 php7.3 php7.3-mcrypt php7.3-bcmath php7.3-bz2 php7.3-cgi php7.3-cli php7.3-common php7.3-curl php7.3-dev php7.3-fpm php7.3-gd php7.3-imap php7.3-intl php7.3-json php7.3-mbstring php7.3-mysql php7.3-opcache php7.3-pspell php7.3-readline php7.3-soap php7.3-sqlite3 php7.3-tidy php7.3-xml php7.3-xmlrpc php7.3-xsl php7.3-zip

Install PHP 7.4 binaries

sudo apt install libonig5 php7.4 php7.4-mcrypt php7.4-bcmath php7.4-bz2 php7.4-cgi php7.4-cli php7.4-common php7.4-curl php7.4-dev php7.4-fpm php7.4-gd php7.4-imap php7.4-intl php7.4-json php7.4-mbstring php7.4-mysql php7.4-opcache php7.4-pspell php7.4-readline php7.4-soap php7.4-sqlite3 php7.4-tidy php7.4-xml php7.4-xmlrpc php7.4-xsl php7.4-zip

Install all PHP versions in one single command

sudo apt install libonig5 php5.6 php5.6-mcrypt php5.6-bcmath php5.6-bz2 php5.6-cgi php5.6-cli php5.6-common php5.6-curl php5.6-dev php5.6-fpm php5.6-gd php5.6-imap php5.6-intl php5.6-json php5.6-mbstring php5.6-mysql php5.6-opcache php5.6-pspell php5.6-readline php5.6-soap php5.6-sqlite3 php5.6-tidy php5.6-xml php5.6-xmlrpc php5.6-xsl php5.6-zip php5.6-bz2 php7.0 php7.0-mcrypt php7.0-bcmath php7.0-bz2 php7.0-cgi php7.0-cli php7.0-common php7.0-curl php7.0-dev php7.0-fpm php7.0-gd php7.0-imap php7.0-intl php7.0-json php7.0-mbstring php7.0-mysql php7.0-opcache php7.0-pspell php7.0-readline php7.0-soap php7.0-sqlite3 php7.0-tidy php7.0-xml php7.0-xmlrpc php7.0-xsl php7.0-zip php7.0-bz2 php7.1 php7.1-mcrypt php7.1-bcmath php7.1-bz2 php7.1-cgi php7.1-cli php7.1-common php7.1-curl php7.1-dev php7.1-fpm php7.1-gd php7.1-imap php7.1-intl php7.1-json php7.1-mbstring php7.1-mysql php7.1-opcache php7.1-pspell php7.1-readline php7.1-soap php7.1-sqlite3 php7.1-tidy php7.1-xml php7.1-xmlrpc php7.1-xsl php7.1-zip php7.1-bz2 php7.2 php7.2-mcrypt php7.2-bcmath php7.2-bz2 php7.2-cgi php7.2-cli php7.2-common php7.2-curl php7.2-dev php7.2-fpm php7.2-gd php7.2-imap php7.2-intl php7.2-json php7.2-mbstring php7.2-mysql php7.2-opcache php7.2-pspell php7.2-readline php7.2-soap php7.2-sqlite3 php7.2-tidy php7.2-xml php7.2-xmlrpc php7.2-xsl php7.2-zip php7.2-bz2 php7.3 php7.3-mcrypt php7.3-bcmath php7.3-bz2 php7.3-cgi php7.3-cli php7.3-common php7.3-curl php7.3-dev php7.3-fpm php7.3-gd php7.3-imap php7.3-intl php7.3-json php7.3-mbstring php7.3-mysql php7.3-opcache php7.3-pspell php7.3-readline php7.3-soap php7.3-sqlite3 php7.3-tidy php7.3-xml php7.3-xmlrpc php7.3-xsl php7.3-zip php7.3-bz2 php7.4 php7.4-mcrypt php7.4-bcmath php7.4-bz2 php7.4-cgi php7.4-cli php7.4-common php7.4-curl php7.4-dev php7.4-fpm php7.4-gd php7.4-imap php7.4-intl php7.4-json php7.4-mbstring php7.4-mysql php7.4-opcache php7.4-pspell php7.4-readline php7.4-soap php7.4-sqlite3 php7.4-tidy php7.4-xml php7.4-xmlrpc php7.4-xsl php7.4-zip php7.4-bz2

Option 2: Install by compiling from source:

If you don't want to use binary packages you can instead compile from source by following these steps:

Download PHP 7.1 source

apt-get install build-essential nano
apt-get install libzip-dev libfcgi-dev libfcgi0ldbl libjpeg62-turbo-dev libmcrypt-dev libssl-dev libc-client2007e libc-client2007e-dev libxml2-dev libbz2-dev libcurl4-openssl-dev libjpeg-dev libpng-dev libfreetype6-dev libkrb5-dev libpq-dev libxml2-dev libxslt1-dev
ln -s /usr/lib/libc-client.a /usr/lib/x86_64-linux-gnu/libc-client.a
cd /usr/include
ln -s x86_64-linux-gnu/curl
mkdir -p /opt/php-7.1
mkdir /usr/local/src/php7.1-build
cd /usr/local/src/php7.1-build
wget http://de2.php.net/get/php-7.1.25.tar.bz2/from/this/mirror -O php-7.1.25.tar.bz2
tar jxf php-7.1.25.tar.bz2

Compile PHP 7.1 from source

./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

Copy default PHP 7.1 ini

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

Configure default PHP 7.1 ini

vi /opt/php-7.1/etc/php-fpm.conf

[...]
pid = run/php-fpm.pid
[...]
vi /opt/php-7.1/etc/php-fpm.d/www.conf
[...]
listen = 127.0.0.1:8999
[...]
vi /lib/systemd/system/php-7.1-fpm.service
[Unit]
Description=The PHP 7.1 FastCGI Process Manager
After=network.target
[Service]
Type=simple
PIDFile=/opt/php-7.1/
var
/run/php-fpm.pid
ExecStart=/opt/php-7.1/sbin/php-fpm --nodaemonize --fpm-config /opt/php-7.1/etc/php-fpm.conf
ExecReload=/bin/kill -USR2
$MAINPID
[Install]
WantedBy=multi-user.target
systemctl enable php-7.1-fpm.service
systemctl daemon-reload
systemctl start php-7.1-fpm.service
vi /opt/php-7.1/lib/php.ini
zend_extension=opcache.so

Test:

cd /opt/php-7.1/bin
./php --version

Enable PHP 7.1 version in ISPConfig

Open FastCGI Settings tab:
Path to the PHP FastCGI binary: /opt/php-7.1/bin/php-cgi
Path to the php.ini directory: /opt/php-7.1/lib
PHP-FPM Settings tab:
Path to the PHP-FPM init script: php-7.1-fpm
Path to the php.ini directory: /opt/php-7.1/lib
Path to the PHP-FPM pool directory: /opt/php-7.1/etc/php-fpm.d