There are two ways to configure Apache to use PHP:

1. Configure Apache to load the PHP interpreter as an Apache module
2. Configure Apache to run the PHP interpreter as a CGI binary

(PS: Windows IIS normaly configures as CGI by the way)

It is the intention of this post to provide you information relating to the configuration and recognition of each method. "In general" historically only one method or the other has been implemented, however, with the architectural changes made to PHP starting with PHP5, it has been quite common for hosting firms to configure for both. One version running as CGI and one version running as a Module. It is generally accepted more recently that running PHP as a CGI is more secure, however, running PHP as an Apache Module does have a slight performance gain and is generally how most pre-configured systems will be delivered out of the box.

What is the difference between CGI and apache Module Mode?

An Apache module is compiled into the Apache binary, so the PHP interpreter runs in the Apache process, meaning that when Apache spawns a child, each process already contains a binary image of PHP. A CGI is executed as a single process for each request, and must make an exec() or fork() call to the PHP executable, meaning that each request will create a new process of the PHP interpreter. Apache is much more efficient in it's ability to handle requests, and managing resources, making the Apache module slightly faster than the CGI (as well as more stable under load).

CGI Mode on the other hand, is more secure because the server now manages and controls access to the binaries. PHP can now run as your own user rather than the generic Apache user. This means you can put your database passwords in a file readable only by you and your php scripts can still access it! The "Group" and "Other" permissions ( refer Permissions FAQ ) can now be more restrictive. CGI mode is also claimed to be more flexible in many respects as you should now not see, with phpSuExec ( refer Permissions under phpSuExec ) issues with file ownership being taken over by the Apache user, therefore you should no-longer have problems under FTP when trying to access or modify files that have been uploaded through a PHP interface, such as Joomla! upload options.

If your server is configured to run PHP as an Apache module, then you will have the choice of using either php.ini or Apache .htaccess files, however, if your server runs PHP in CGI mode then you will only have the choice of using php.ini files locally to change settings, as Apache is no longer in complete control of PHP.

Testing and Reviewing Your PHP Installation

Also known as "Everything you ever wanted and didn't want to know about PHP"

To find out the PHP interpreter mode and to generally test your PHP installation and to find out a vast amount of information about your PHP environment, supported utilities, applications and settings, you create a single PHP file containing only the following lines;

phpinfo();

This single line of code outputs an amazing amount of information, be warned....

Save the file as any filename you wish, but with the ".php" extension. FTP it to your server and open it in a browser.

Other useful information

The following are PHP functions, that when run from a PHP File can provide some useful information, (less than the above option) many should run on most hosts, however many hosts disable some of these functions for security. No Guarantee's offered...

Again, as above, make a file, name it anything you wish but make sure it has the ".php" extension, copy and paste the following lines in to it and FTP to your server.

echo "Hostname: ". @php_uname(n) ."";
if (function_exists( 'shell_exec' )) { echo "Hostname: ".
@gethostbyname(trim(`hostname`)); } else { echo "Server IP: ".
$_SERVER['SERVER_ADDR'] .""; }
echo "Platform: ". @php_uname(s) ." ". @php_uname(r) ." ". @php_uname(v) ."";
echo "Architecture: ". @php_uname(m) ."";
echo "Username: ". get_current_user () ." ( UiD: ". getmyuid() .", GiD: ". getmygid() ." )";
echo "Curent Path: ". getcwd () ."";
echo "Server Type: ". $_SERVER['SERVER_SOFTWARE'] . "";
echo "Server Admin: ". $_SERVER['SERVER_ADMIN'] . "";
echo "Server Signature: ". $_SERVER['SERVER_SIGNATURE'] ."";
echo "Server Protocol: ". $_SERVER['SERVER_PROTOCOL'] ."";
echo "Server Mode: ". $_SERVER['GATEWAY_INTERFACE'] ."";
?>

The Joomla! HISA or Joomla! Tools Suite can also assist to determine which mode your server in running in, also providing a large amount of other related information including recommendations on configuration;

Joomla! Tools Suite (JTS) is a complete "Suite" of Tools to help you troubleshoot and maintain Joomla! and include the "HISA" script
Download JTS Here

Joomla! Health, Installation and Security Audit (HISA) is a single standalone script that provides purely configuration information.
Download HISA Here

Forum Discussion Here
How to TroubleShoot A Joomla! Installation

Another Indirect method, and possibly not 100% reliable, is that if you are unable to make use of .htaccess on Linux hosting and Apache based servers then you are either running in CGI mode or your host has disabled the use of .htaccess even if your server is running PHP as an Apache Module.

Remove these files immediately after use, the information contained in their output is extensive and explicit regarding your PHP and server configurations, it will help those wishing to cause your site harm

For those wishing to know more about "How To..."

Running PHP as an Apache module
To configure Apache to load PHP as a module to 'parse' your PHP scripts, the httpd.conf needs to be modified, typically found in "c:\Program Files\Apache Group\Apache\conf\" or "/etc/httpd/conf/".

Search for the section of the file that has a series of commented out "LoadModule" statements. (Statements prefixed by the hash "#" sign are regarded as having been commented out.) If PHP is running in "Apache Module" Mode you should see something very similar to the following;

LoadModule php4_module "c:/php/php4apache.dll"

Apache 1.x

For PHP5

LoadModule php5_module     C:/php/php5apache2.dll
or (platform dependant)
LoadModule php5_module     /usr/lib/apache/libphp5.so

For PHP4

LoadModule php4_module     libexec/libphp4.so
or (platform dependant)

LoadModule php4_module C:/php/php4apache.dll

and

AddModule mod_php4.c

or

AddModule mod_php5.c

Apache 2.x

For PHP5

LoadModule php5_module     C:/php/php5apache2.dll

or (platform dependant)

LoadModule php5_module     /usr/lib/apache/libphp5.so

For PHP4

LoadModule php4_module     libexec/libphp4.so

or (platform dependant)
LoadModule php4_module C:/php/php4apache.dll

and
AddModule mod_php5.c

or
AddModule mod_php4.c

Note:
Don't worry that you can't find a "mod_php4.c" or "mod_php5.c" file anywhere on your system. That directive does not cause Apache to search for the file on your system. For the curious, it specifies the order in which the various modules are enabled by the Apache server.

If you're using Apache 2.x, you do not have to insert the AddModule directive. It's no longer needed in that version. Apache 2.x has its own internal method of determining the correct order of loading the modules.

Now find the "AddType" section in the file, and add the following line after the last "AddType" statement:

AddType application/x-httpd-php .php

If you need to support other file types, like ".php3" and ".phtml", simply add them to the list, like this:<

AddType application/x-httpd-php .php3
AddType application/x-httpd-php .phtml

Run a syntax check and if all is ok, restart Apache...

Running PHP as a CGI binary
To configure PHP to run as a CGI, again you will need to configure the httpd.conf, but confirm that the above settings are not also configured, unless you now what you are doing you can generate yourself "HTTP 500" errors. Search your Apache configuration file for the "ScriptAlias" section.

Add the following line below after the ScriptAlias for "cgi-bin".

Note:

The location will depend on where PHP is installed on your system, you should substitute the appropriate path in place of "c:/php/" (for example, "c:/Program Files/php/").

ScriptAlias /php/ "c:/php/"

Apache again needs to be configured for the PHP MIME type. Search for the "AddType" section, and add the following line after it:

AddType application/x-httpd-php .php

As in the case of running PHP as an Apache module, you can add whatever extensions you want Apache to recognise as PHP scripts, such as:

AddType application/x-httpd-php .php3
AddType application/x-httpd-php .phtml


Next, you will need to tell the server to execute the PHP executable each time it encounters a PHP script. Add the following below any existing entries in the "Action" section.

Action application/x-httpd-php "/php/php.exe"

If you notice, we have used the "ScriptAlias" reference, "/php/" portion will be recognised as the scriptAlias configured above, this is sort a path alias which will correlate to your PHP installation path configured previously. In other words, don't put "c:/php/php.exe" or "c:/Program Files/php/php.exe" in that directive, put

"/php/php.exe", Apache WILL work it out if correctly configured.

Configuring the Default Index Page
This section applies to all users, whether you are loading PHP as a module or running it as a CGI binary, and has been seen often enough to warrant a mention.

If you want to make your PHP script execute as the default page for a directory, you have to add another line to the "httpd.conf". Simply search for the line in the file that begins with a "DirectoryIndex" and add "index.php" to the list of files on that line. For example, if the line used to be:

DirectoryIndex index.html

change it to

DirectoryIndex index.html index.php
If you still wish .html files to be executed before .php files

or
DirectoryIndex index.php index.html
If you wish .php files to be executed before .html files

The next time you access the site or a directory within a site without a filename, Apache will "auto-magically" deliver "index.php" if available, or "index.html" if "index.php" is not available.