What is mod_php?

Your server needs to have the php modules installed so it can parse php code.

If you are on ubuntu you can do this easily with

sudo apt-get install apache2

sudo apt-get install php5

sudo apt-get install libapache2-mod-php5

sudo /etc/init.d/apache2 restart

Otherwise you may compile apache with php: http://dan.drydog.com/apache2php.html

Specifying your server OS will help others to answer more specifically.

This answer is taken from TuxRadar:

When running PHP through your web server, there are two distinct options: running it using PHP's CGI SAPI, or running it as a module for the web server. Each have their own benefits, but, overall, the module is generally preferred.

Running PHP as a CGI means that you basically tell your web server the location of the PHP executable file, and the server runs that executable, giving it the script you called, each time you visit a page. That means each time you load a page, PHP needs to read php.ini and set its settings, it needs to load all its extensions, and then it needs to start work parsing the script - there is a lot of repeated work.

When you run PHP as a module, PHP literally sits inside your web server - it starts only once, loads its settings and extensions only once, and can also store information across sessions. For example, PHP accelerators rely on PHP being able to save cached data across requests, which is impossible using the CGI version.

The obvious advantage of using PHP as a module is speed - you will see a big speed boost if you convert from CGI to a module. Many people, particularly Windows users, do not realise this, and carry on using the php.exe CGI SAPI, which is a shame - the module is usually three to five times faster.

There is one key advantage to using the CGI version, though, and that is that PHP reads its settings every time you load a page. With PHP running as a module, any changes you make in the php.ini file do not kick in until you restart your web server, which makes the CGI version preferable if you are testing a lot of new settings and want to see instant responses.

mod_php means PHP, as an Apache module.

Basically, when loading mod_php as an Apache module, it allows Apache to interpret PHP files (those are interpreted by mod_php).

EDIT : There are (at least) two ways of running PHP, when working with Apache :

• Using CGI : a PHP process is launched by Apache, and it is that PHP process that interprets PHP code -- not Apache itself
• Using PHP as an Apache module (called mod_php) : the PHP interpreter is then kind of "embedded" inside the Apache process : there is no external PHP process -- which means that Apache and PHP can communicate better.

And re-edit, after the comment : using CGI or mod_php is up to you : it's only a matter of configuration of your webserver.

To know which way is currently used on your server, you can check the output of phpinfo() : there should be something indicating whether PHP is running via mod_php (or mod_php5), or via CGI.

You might also want to take a look at the php_sapi_name() function : it returns the type of interface between web server and PHP.

If you check in your Apache's configuration files, when using mod_php, there should be a LoadModule line looking like this :

LoadModule php5_module        modules/libphp5.so

(The file name, on the right, can be different -- on Windows, for example, it should be a .dll)

Just to add on these answers is that, mod_php is the oldest and slowest method available in HTTPD server to use PHP. It is not recommended, unless you are running old versions of Apache HTTPD and PHP. PHP-FPM and proxy_cgi are the preferred methods.