FastCGI is a protocol for interfacing interactive programs with a web server. FastCGI’s main aim is to reduce the overhead associated with interfacing the web server and CGI programs, allowing a server to handle more web page requests at once.

Also, PHP is not recommended with multithreaded Apache2 (worker MPM) because of performance and some 3rd party PHP extensions are not not guaranteed thread-safe.

nginx and lighttpd has inbuilt support for FastCGI. For Apache web server you need to use either mod_fastcgi or mod_fcgid.

Why use mod_fastcgi instead of mod_perl / mod_php?

From the wikipedia article:

Instead of creating a new process for every request, FastCGI can use a single persistent process which handles many requests over its lifetime. Processing of multiple requests simultaneously is achieved either by using a single connection with internal multiplexing (ie. multiple requests over a single connection) and/or by using multiple connections. Many such processes can exist, something that can increase stability and scalability. FastCGI also allows programs to get the web server to do certain simple operations, like reading in a file, before the request is handed over. Environment information and page requests are sent from the web server to the process over a TCP connection (for remote processes) or Unix domain sockets (for local processes). Responses are returned from the process to the web server over the same connection. The connection may be closed at the end of a response, but the web server and the process are left standing.

Many web site administrators and programmers are finding that the separation of web applications from the web server in FastCGI (and the simpler SCGI) has many desirable advantages over embedded interpreters (mod_perl, mod_php, etc.). This separation allows server and application processes to be restarted independently — an important consideration for busy web sites. It also facilitates per-application security policies — important for ISPs and web hosting companies.

In this quick tutorial, you will learn about Apache 2 + mod_fastcgi + PHP installation and configuration under Red Hat Enterprise Linux / CentOS Linux version 5.x+.

Install mod_fastcgi

Make sure required packages are installed (httpd-devel and apr-devel required to compile mod_fastcgi), enter:
# yum install libtool httpd-devel apr-devel apr
Next, grab the latest mod_fastcgi source code, enter:
# cd /opt
# wget http://www.fastcgi.com/dist/mod_fastcgi-current.tar.gz

Untar tar ball, type:
# tar -zxvf mod_fastcgi-current.tar.gz
# cd mod_fastcgi-2.4.6/
Make a copy of Makefile.AP2, enter:
# cp Makefile.AP2 Makefile
Compile and install mod_fastcgi for 32 bit system, enter:
# make top_dir=/usr/lib/httpd
# make install top_dir=/usr/lib/httpd

Compile and install mod_fastcgi for 64 bit system, enter:
# make top_dir=/usr/lib64/httpd
# make install top_dir=/usr/lib64/httpd
Sample output:

make install top_dir=/usr/lib64/httpd
make[1]: Entering directory `/tmp/mod_fastcgi-2.4.6'
/usr/lib64/apr-1/build/libtool --silent --mode=install cp mod_fastcgi.la /usr/lib64/httpd/modules/
make[1]: Leaving directory `/tmp/mod_fastcgi-2.4.6'

Configure mod_fastcgi

Open /etc/httpd/conf.d/mod_fastcgi.conf file
# vi /etc/httpd/conf.d/mod_fastcgi.conf
Add an entry to it like this:
LoadModule fastcgi_module modules/mod_fastcgi.so
Save and close the file. Restart httpd, enter:
# service httpd restart
Sample output:

[Mon Dec 29 23:24:44 2008] [notice] caught SIGTERM, shutting down
[Mon Dec 29 23:24:44 2008] [notice] suEXEC mechanism enabled (wrapper: /usr/sbin/suexec)
[Mon Dec 29 23:24:44 2008] [notice] Digest: generating secret for digest authentication ...
[Mon Dec 29 23:24:44 2008] [notice] Digest: done
[Mon Dec 29 23:24:44 2008] [notice] FastCGI: process manager initialized (pid 4785)
[Mon Dec 29 23:24:44 2008] [notice] Apache/2.2.3 (CentOS) configured -- resuming normal operations

How do I configure PHP as FastCGI process under RHEL / CentOS Linux?

First, you need to disable mod_php5, enter:
# mv /etc/httpd/conf.d/php.conf /etc/httpd/conf.d/php.conf.disable

Create a shell script in cgi-bin directory called php.fcgi

Create a script as follows in /var/www/cgi-bin/php.fcgi (or put in your virtual domain cgi-bin directory)

#!/bin/bash
# Shell Script To Run PHP5 using mod_fastcgi under Apache 2.x
# Tested under Red Hat Enterprise Linux / CentOS 5.x
### Set PATH ###
PHP_CGI=/usr/bin/php-cgi
PHP_FCGI_CHILDREN=4
PHP_FCGI_MAX_REQUESTS=1000
### no editing below ###
export PHP_FCGI_CHILDREN
export PHP_FCGI_MAX_REQUESTS
exec $PHP_CGI

Set permission, type:
# chmod +x /var/www/cgi-bin/php.fcgi
Finally, modify documentroot directory permission as follows. You need to use AddHandler and Action directives for mod_fastcgi:

 <Directory "/var/www/html">
   Options -Indexes FollowSymLinks +ExecCGI
    AllowOverride AuthConfig FileInfo
    AddHandler php5-fastcgi .php
    Action php5-fastcgi /cgi-bin/php.fcgi
    DirectoryIndex index.php index.html
    Order allow,deny
    Allow from all
</Directory>

Where,

• AddHandler php5-fastcgi .php : To configure Apache to handle php files (within the scope of the directive) with the specified extension(s) as FastCGI applications.
• Action php5-fastcgi /cgi-bin/php.fcgi : This directive adds an action, which will activate cgi-script when action-type is triggered by the request. The cgi-script is the URL-path to a resource that has been configured as a CGI script using ScriptAlias. In our case, requests for files with a .php file extension of are handled instead by the specified cgi script /cgi-bin/php.fcgi.

Save and close the file. Restart httpd:
# service httpd restart

mod_fastcgi virtual hosting configuration

• Domain name: nixcraft.com
• DocumentRoot: /websites/home/nixcraft.com/http
• cgi-bin directory: /websites/home/nixcraft.com/cgi-bin
• php.fcgi path: /websites/home/nixcraft.com/cgi-bin/php.fcgi
• Logs file directory: /websites/home/nixcraft.com/logs

Based upon above settings your virtualhosting configuration for nixcraft.com domain should look like as follows:

<VirtualHost *:80>
        ServerAdmin webmaster@nixcraft.com
        DocumentRoot "/websites/home/nixcraft.com/http"
        ServerName nixcraft.com
        ServerAlias www.nixcraft.com
        ErrorLog "/websites/home/nixcraft.com/logs/error.log"
        CustomLog "/websites/home/nixcraft.com/logs/access.log" common
        ScriptAlias /cgi-bin/ "/websites/home/nixcraft.com/cgi-bin/"
 
	<Directory "/websites/home/nixcraft.com/http">
		Options -Indexes FollowSymLinks +ExecCGI
		AllowOverride AuthConfig FileInfo
		AddHandler php5-fastcgi .php
		Action php5-fastcgi /cgi-bin/php.fcgi
		Order allow,deny
		Allow from all
	</Directory>
 
	<Directory "/websites/home/nixcraft.com/cgi-bin">
		AllowOverride None
		Options None
		Order allow,deny
		Allow from all
	</Directory>
</VirtualHost>

Create a /websites/home/nixcraft.com/cgi-bin/php.fcgi as follows:

#!/bin/bash
PHP_CGI=/usr/bin/php-cgi
PHP_FCGI_CHILDREN=4
PHP_FCGI_MAX_REQUESTS=1000
export PHP_FCGI_CHILDREN
export PHP_FCGI_MAX_REQUESTS
exec $PHP_CGI

Set permission and restart httpd:
# chmod +x /websites/home/nixcraft.com/cgi-bin/php.fcgi
# service httpd restart