1st of all check that PHP5-FPM and suexec are installed on your server, if not you can do this AFTER installing ISPConfig

apt-get install apache2-mpm-worker apache2-suexec php5-cgi libapache2-mod-fcgid 
a2enmod fcgid
a2enmod suexec

2 – Install HHVM

sudo add-apt-repository ppa:mapnik/boost
wget -O – http://dl.hhvm.com/conf/hhvm.gpg.key | sudo apt-key add -
echo deb http://dl.hhvm.com/ubuntu precise main | sudo tee /etc/apt/sources.list.d/hhvm.list
sudo apt-get update
sudo apt-get install hhvm

3 – ISPConfig Modifications

in /interface/web/sites/form/web_domain.tform.php

Add :

'hhvm' => 'HHVM Over PHP-FPM'

In the ‘value’ sub-array of this block :

      'php' => array (
            'datatype'    => 'VARCHAR',
            'formtype'    => 'SELECT',
            'default'    => 'fast-cgi',
            'valuelimit' => 'client:web_php_options',
            'value'        => array('no' => 'disabled_txt', 'fast-cgi' => 'Fast-CGI', 'cgi' => 'CGI', 'mod' => 'Mod-PHP', 'suphp' => 'SuPHP', 'php-fpm' => 'PHP-FPM'),
            'searchable' => 2
        )

so the line will look like :

'value'        => array('no' => 'disabled_txt', 'fast-cgi' => 'Fast-CGI', 'cgi' => 'CGI', 'mod' => 'Mod-PHP', 'suphp' => 'SuPHP', 'php-fpm' => 'PHP-FPM', 'hhvm' => 'HHVM Over PHP-FPM'),

4 – in /server/conf/vhost.conf.master

after :

after <tmpl_if name='php' op='==' value='php-fpm'> <IfModule mod_fastcgi.c>

Add :

<tmpl_if name='php' op='==' value='hhvm'>
    <IfModule mod_fastcgi.c>
        <Directory {tmpl_var name='document_root'}/cgi-bin>
            Order allow,deny
            Allow from all
        </Directory>
                AddHandler php5-fcgi .php
                Action php5-fcgi /php5-fcgi
        Alias /php5-fcgi {tmpl_var name='document_root'}/cgi-bin/php5-fcgi-{tmpl_var name='ip_address'}-{tmpl_var name='port'}-{tmpl_var name='domain'}

        <tmpl_if name='use_tcp'>
                    FastCgiExternalServer {tmpl_var name='document_root'}/cgi-bin/php5-fcgi-{tmpl_var name='ip_address'}-{tmpl_var name='port'}-{tmpl_var name='domain'} -idle-timeout 300 -host 127.0.0.1:<tmpl_var name='hhvm_port'> -pass-header Authorization
        </tmpl_if>
        <tmpl_if name='use_socket'>
                    FastCgiExternalServer {tmpl_var name='document_root'}/cgi-bin/php5-fcgi-{tmpl_var name='ip_address'}-{tmpl_var name='port'}-{tmpl_var name='domain'} -idle-timeout 300 -socket <tmpl_var name='hhvm_socket'> -pass-header Authorization
        </tmpl_if>
    </IfModule>
</tmpl_if>

Change this :

<tmpl_if name='php' op='==' value='php-fpm'>
    DocumentRoot <tmpl_var name='web_document_root'>
</tmpl_else>
    DocumentRoot <tmpl_var name='web_document_root_www'>
</tmpl_if>

into this :

<tmpl_if name='php' op='==' value='php-fpm'>
    DocumentRoot <tmpl_var name='web_document_root'>
</tmpl_else>
    <tmpl_if name='php' op='==' value='hhvm'>
        DocumentRoot <tmpl_var name='web_document_root'>
    </tmpl_else>
        DocumentRoot <tmpl_var name='web_document_root_www'>
    </tmpl_if>
</tmpl_if>

5 –  server/plugins-avaialble/apache2_plugin.inc.php

add this method inside the class :

function hhvm_upstart($web_config){
    global $app,$conf;
    // create upstart script

        //clear service_pool in mysql

        $script_content = '#!/bin/bash

mysql –user="'.$conf['db_user'].’” –password=”‘.$conf['db_password'].’” –database=”‘.$conf['db_database'].'" –execute="DELETE FROM service_pool"'."nn";

        $upstart_services = $app->db->queryAllRecords("SELECT * FROM web_domain WHERE type='vhost' and php = 'hhvm'");
        file_put_contents('/tmp/isp/t.txt', print_r($upstart_services,true));
        if(is_array($upstart_services) && !empty($upstart_services)){
            foreach ($upstart_services as $service){
                $script_content .= 'cd '.$service['document_root']."n";

                $port = '2' . ($web_config['php_fpm_start_port'] + $service['domain_id'] – 1); // get the hhvm port
                $script_content .= 'hhvm –mode server -vServer.Type=fastcgi -vServer.Port=' . $port . ' > /dev/null 2>&1 & PID=$!; '."n";
                $script_content .= 'mysql –user="'.$conf['db_user'].’” –password=”‘.$conf['db_password'].’” –database=”‘.$conf['db_database'].’” –execute=”INSERT INTO service_pool (service_type,server_id,service_pid) VALUES (’hhvm’ ,”.$service['domain_id'].'', '$PID')"'."n";

            }
        }

        file_put_contents('/etc/init.d/hhvm_ispconfig.sh', $script_content);
        exec('chmod +x /etc/init.d/hhvm_ispconfig.sh');
        exec('update-rc.d hhvm_ispconfig.sh defaults');
}

add this line to the end of update() method :

$this->hhvm_upstart($web_config);

After :

$fpm_socket = $socket_dir.$pool_name.'.sock';
$tpl->setVar('fpm_socket', $fpm_socket);
$tpl->setVar('fpm_port', $web_config['php_fpm_start_port'] + $data['new']['domain_id'] – 1);

Add :

$hhvm_socket = $socket_dir.$pool_name.'.hhvm.sock';
$tpl->setVar('hhvm_socket', $hhvm_socket);
$tpl->setVar('hhvm_port', '2' . ($web_config['php_fpm_start_port'] + $data['new']['domain_id'] – 1));

Replace :

if($data['new']['php'] == 'php-fpm'){

With :

if($data['new']['php'] == ‘php-fpm’ || $data['new']['php'] == 'hhvm'){

Replace :

if(trim($data['old']['fastcgi_php_version']) != ” && $data['old']['php'] == 'php-fpm'){

With :

if(trim($data['old']['fastcgi_php_version']) != ” && ($data['old']['php'] == ‘php-fpm’  || $data['new']['php'] == 'hhvm')){

Replace :

if(trim($data['old']['fastcgi_php_version']) != ” && $data['old']['php'] == 'php-fpm'){

With :

if(trim($data['old']['fastcgi_php_version']) != ” && ($data['old']['php'] == ‘php-fpm’  || $data['new']['php'] == 'hhvm')){

in the end of method « php_fpm_pool_update » add :

    //Remove all hhvm instances
    $old_services = $app->db->queryAllRecords("SELECT * FROM service_pool WHERE service_type = 'hhvm' AND server_id = '".$data['new']['domain_id']."'");
    if(is_array($old_services) && !empty($old_services)){
        foreach ($old_services as $old_service){
        exec("kill -9 ".$old_service['service_pid']);
        }
    }
    $app->db->query("DELETE FROM service_pool WHERE service_type = 'hhvm' AND server_id = '".$data['new']['domain_id']."'");

    $tmpCurDir = __DIR__; // store the current cirectory
    chdir($data['new']['document_root']); // change to the document root as HHVM needs to be run from there

    // start a fresh hhvm instance
    $port = '2' . ($web_config['php_fpm_start_port'] + $data['new']['domain_id'] – 1); // get the hhvm port
    $command =  'hhvm –mode server -vServer.Type=fastcgi -vServer.Port=' . $port . ' > /dev/null 2>&1 & echo $!; ';
    $pid = exec($command, $output);

    $app->db->query("INSERT INTO service_pool (service_type,server_id,service_pid) VALUES ('hhvm' ,'".$data['new']['domain_id']."', '".$pid."')");

6 – in /interface/web/sites/templates/web_domain_edit.htm

Replace :

if(jQuery('#php').val() == 'fast-cgi' || jQuery('#php').val() == 'php-fpm'){

With :

if(jQuery('#php').val() == 'fast-cgi' || jQuery('#php').val() == 'php-fpm' || jQuery('#php').val() == 'hhvm'){

Replace :

if(jQuery(this).val() == 'fast-cgi' || jQuery(this).val() == 'php-fpm'){

With :

if(jQuery(this).val() == 'fast-cgi' || jQuery(this).val() == 'php-fpm' || jQuery(this).val() == 'hhvm'){

7 – Create the following table in the ISPConfig database :

CREATE TABLE IF NOT EXISTS `service_pool` (
  `service_id` int(11) NOT NULL AUTO_INCREMENT,
  `service_type` varchar(32) NOT NULL,
  `service_pid` int(11) NOT NULL,
  `server_id` int(11) NOT NULL,
  PRIMARY KEY (`service_id`),
  KEY `service_type` (`service_type`),
  KEY `server_id` (`server_id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

Hopy this will help you out guys ! Enjoy !

sources : http://blog.noczone.com/?p=102