How to select PHP version 5 and 7 per virtualhost in Apache 2.4 on Debian?

Let's start from beginning. I assume that you would prefer to use php-fpm instead of Apache module.

First install apache:

sudo apt-get updatesudo apt-get install apache2

Next install multiple PHP:

Debian 9:
Install PHP 7:

sudo apt-get install php7.0-cli php7.0-fpm php-pear libapache2-mod-fastcgi

Configure repositories:

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

Install PHP 5:

sudo apt-get install php5.6-cli php5.6-fpm

Debian 8:
Install PHP 5:

sudo apt-get install php5 php5-fpm php-pear libapache2-mod-fastcgi

Configure repositories:
Edit /etc/apt/sources.list and add the following lines to the end of file:

deb http://packages.dotdeb.org jessie alldeb-src http://packages.dotdeb.org jessie all

Install GPG key:

wget https://www.dotdeb.org/dotdeb.gpgsudo apt-key add dotdeb.gpgsudo apt-get update

Install PHP 7:

sudo apt-get install php7.0 php7.0-fpm

Next switch from prefork and enable necessary modules:
For Debian 8:

a2dismod php5 mpm_prefork

For Debian 9:

a2dismod php7 mpm_prefork

Next for both:

a2enmod actions fastcgi alias proxy_fcgi mpm_workersystemctl restart apache2

Change content of /etc/apache2/mods-enabled/fastcgi.conf to the following one:

<IfModule !mod_fastcgi.c>    AddHandler fcgid-script fcg fcgi fpl</IfModule><IfModule mod_fastcgi.c>    <Directory /usr/lib/cgi-bin>        Require all granted    </Directory></IfModule>

Now create document root folders for websites:

mkdir -p /var/www/example.com/public_htmlmkdir -p /var/www/test.com/public_html

Add sys users for these websites:

sudo useradd example --home-dir /var/www/example.comsudo useradd test --home-dir /var/www/test.com

Configure ownership:

sudo chown -R example.example /var/www/example.comsudo chown -R test.test /var/www/test.com

For example website example.com will use PHP 5 and website test.com will use PHP 7.

Create configuration files for websites:
Website on PHP 5:

touch /etc/apache2/sites-available/example.com.confln -s /etc/apache2/sites-available/example.com.conf /etc/apache2/sites-enabled/example.com.confcat /etc/apache2/sites-available/example.com.conf<VirtualHost *:80>        ServerAdmin webmaster@localhost        ServerName example.com        ServerAlias www.example.com        DocumentRoot /var/www/example.com/public_html        ErrorLog ${APACHE_LOG_DIR}/error.log        CustomLog ${APACHE_LOG_DIR}/access.log combined        <IfModule mod_fastcgi.c>            AddType application/x-httpd-fastphp5 .php            Action application/x-httpd-fastphp5 /php5-fcgi            Alias /php5-fcgi /usr/lib/cgi-bin/php5-fcgi-example.com            FastCgiExternalServer /usr/lib/cgi-bin/php5-fcgi-example.com -socket /var/run/php5-fpm-example.com.sock -pass-header Authorization        </IfModule></VirtualHost>

Website on PHP 7:

touch /etc/apache2/sites-available/test.com.confln -s /etc/apache2/sites-available/test.com.conf /etc/apache2/sites-enabled/test.com.confcat /etc/apache2/sites-available/test.com.conf<VirtualHost *:80>        ServerAdmin webmaster@localhost        ServerName test.com        ServerAlias www.test.com        DocumentRoot /var/www/test.com/public_html        ErrorLog ${APACHE_LOG_DIR}/error.log        CustomLog ${APACHE_LOG_DIR}/access.log combined        <IfModule mod_fastcgi.c>                AddHandler php7-fcgi .php                Action php7-fcgi /php7-fcgi virtual                Alias /php7-fcgi /usr/lib/cgi-bin/php7-fcgi-test.com                FastCgiExternalServer /usr/lib/cgi-bin/php7-fcgi-test.com -socket /var/run/php/php7.0-fpm-test.com.sock -pass-header Authorization        </IfModule></VirtualHost>

Create pool configs (I used the following):
Website on PHP 5:

cat /etc/php5/fpm/pool.d/example.com.conf[example.com]user = examplegroup = examplelisten = /var/run/php5-fpm-example.com.socklisten.owner = www-datalisten.group = www-dataphp_admin_value[disable_functions] = exec,passthru,shell_exec,systemphp_admin_flag[allow_url_fopen] = offpm = dynamicpm.max_children = 5pm.start_servers = 2pm.min_spare_servers = 1pm.max_spare_servers = 3chdir = /

Website on PHP 7:

cat /etc/php/7.0/fpm/pool.d/test.com.conf[test.com]user = testgroup = testlisten = /var/run/php/php7.0-fpm-test.com.socklisten.owner = www-datalisten.group = www-dataphp_admin_value[disable_functions] = exec,passthru,shell_exec,systemphp_admin_flag[allow_url_fopen] = offpm = dynamicpm.max_children = 5pm.start_servers = 2pm.min_spare_servers = 1pm.max_spare_servers = 3chdir = /

Restart apache and php-fpm services:

sudo systemctl restart apache2 php5-fpm php7.0-fpm

Enjoy!

The answer by @elvis-plesky is really detailled. But Apache suggests the usage of mod_proxy_fcgi:

With the release of apache httpd 2.4 upon an unsuspecting populace, we have gained some very neat functionality regarding apache and php: the ability to run PHP as a fastCGI process server, and address that fastCGI server directly from within apache, via a dedicated proxy module (mod_proxy_fcgi.)

Here are the steps to setup two vhosts with different PHP versions for Debian 10 using Apache's mod_proxy_fcgi and php-fpm.

Configure repositories:

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

Install needed PHP versions:

sudo apt-get install php5.6-cli php5.6-fpm php7.3-cli php7.3-fpm

Configure php5.6-fpm in /etc/apache2/conf-available/php5.6-fpm.conf:

<IfModule !mod_php5.c><IfModule proxy_fcgi_module>    # Enable http authorization headers    <IfModule setenvif_module>        SetEnvIfNoCase ^Authorization$ "(.+)" HTTP_AUTHORIZATION=$1    </IfModule>    <FilesMatch ".+\.ph(ar|p|tml)$">        SetHandler "proxy:unix:/run/php/php5.6-fpm.sock|fcgi://php56.localhost"    </FilesMatch>    # Deny access to files without filename (e.g. '.php')    <FilesMatch "^\.ph(ar|p|ps|tml)$">        Require all denied    </FilesMatch></IfModule></IfModule>

Configure php7.3-fpm in /etc/apache2/conf-available/php7.3-fpm.conf:

<IfModule !mod_php7.c><IfModule proxy_fcgi_module>    # Enable http authorization headers    <IfModule setenvif_module>        SetEnvIfNoCase ^Authorization$ "(.+)" HTTP_AUTHORIZATION=$1    </IfModule>    <FilesMatch ".+\.ph(ar|p|tml)$">        SetHandler "proxy:unix:/run/php/php7.3-fpm.sock|fcgi://php73.localhost"    </FilesMatch>    # Deny access to files without filename (e.g. '.php')    <FilesMatch "^\.ph(ar|p|ps|tml)$">        Require all denied    </FilesMatch></IfModule></IfModule>

Now add a virtual host that uses PHP 5.6:

<VirtualHost *:80>    ServerName test-php56.localhost    DocumentRoot "/var/www/test-php56/"    # use php 5.6    <FilesMatch ".+\.ph(ar|p|tml)$">        SetHandler "proxy:unix:/run/php/php5.6-fpm.sock|fcgi://php56.localhost"    </FilesMatch></VirtualHost>

And a virtual host that uses PHP 7.3:

<VirtualHost *:80>    ServerName test-php73.localhost    DocumentRoot "/var/www/test-php73/"    # use php 7.3    <FilesMatch ".+\.ph(ar|p|tml)$">        SetHandler "proxy:unix:/run/php/php7.3-fpm.sock|fcgi://php73.localhost"    </FilesMatch></VirtualHost>

Activate the correct Apache module and the configs:

sudo a2enmod proxy_fcgisudo a2enconf php5.6-fpmsudo a2enconf php7.3-fpmsudo systemctl restart apache2 php5.6-fpm php7.3-fpm

The default configuration of the FPM pools should be correct already.