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

Would it be possible to run PHP 7 and PHP 5 simultaneously in Apache 2.4 on Debian 9? I would like to be able to select the PHP version I wish to use per virtualhost. I believe this would be useful considering that some of my websites still use deprecated PHP features. This allows me to perform upgrades per site. How do I achieve something like this?

For example

<VirtualHost *:80>
   ServerAdmin webmaster@localhost
   ServerName mywebsite.com
   DocumentRoot /var/www/mywebsite.com

   # UsePHP 7
</virtualHost>

And

<VirtualHost *:80>
   ServerAdmin webmaster@localhost
   ServerName mywebsite2.com
   DocumentRoot /var/www/mywebsite2.com

   # UsePHP 5
</virtualHost>

Solution

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 update
sudo 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-https
sudo 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.list
sudo 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 all
deb-src http://packages.dotdeb.org jessie all

Install GPG key:

wget https://www.dotdeb.org/dotdeb.gpg
sudo apt-key add dotdeb.gpg
sudo 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_worker
systemctl 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_html
mkdir -p /var/www/test.com/public_html

Add sys users for these websites:

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

Configure ownership:

sudo chown -R example.example /var/www/example.com
sudo 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.conf
ln -s /etc/apache2/sites-available/example.com.conf /etc/apache2/sites-enabled/example.com.conf
cat /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.conf
ln -s /etc/apache2/sites-available/test.com.conf /etc/apache2/sites-enabled/test.com.conf
cat /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 = example
group = example
listen = /var/run/php5-fpm-example.com.sock
listen.owner = www-data
listen.group = www-data
php_admin_value[disable_functions] = exec,passthru,shell_exec,system
php_admin_flag[allow_url_fopen] = off
pm = dynamic
pm.max_children = 5
pm.start_servers = 2
pm.min_spare_servers = 1
pm.max_spare_servers = 3
chdir = /

Website on PHP 7:

cat /etc/php/7.0/fpm/pool.d/test.com.conf
[test.com]
user = test
group = test
listen = /var/run/php/php7.0-fpm-test.com.sock
listen.owner = www-data
listen.group = www-data
php_admin_value[disable_functions] = exec,passthru,shell_exec,system
php_admin_flag[allow_url_fopen] = off
pm = dynamic
pm.max_children = 5
pm.start_servers = 2
pm.min_spare_servers = 1
pm.max_spare_servers = 3
chdir = /

Restart apache and php-fpm services:

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

Enjoy!