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The Git Submodule facility is really Great for re-using code from other projects. By modularizing the code and keeping things in possibly separate repositories, you have the ability then to mix and match for your application as you see fit.

Modularization in general is a great thing usually, as well as writing code that can be reused. Even if you aren't doing OOP, you can still write many good utilities so you don't have to reinvent the wheel. Create something with a general interface and function and take it with you.

The submodule facility is loosely explained in the Git Book although the explanation still leaves something to desire. Here are the steps I took to do this, with specific examples.

First, my php common code is in a repository on http://gitorious.org at:

  https://gitorious.org/common-php-code

The public URL for getting the code is:

  https://git.gitorious.org/common-php-code/common-php-code.git

So, first I created my super project, in this case earthquakewatch, initialized it and made an initial commit. I created a public repository this as well at gitorious:

  https://gitorious.org/earthquakwatch/earthquakwatch

(Yes, I know it's misspelled)

The public URL for this project is:

  https://git.gitorious.org/earthquakwatch/earthquakwatch.git

Okay, the structure of this project is:

  APP_ROOT/
  |
  +-css/       	       	       	-- styling stuff
  | |
  | + style.css
  | + images/
  |
  +-docs/			-- application documentation
  | +-sample-config.php	       	-- sample configuration file to customize
  |
  +-index.php  	       	       	-- application main file
  |
  +-js/		       		-- javascript stuff
  | +-jquery.js
  +-lib/       			-- libraries
  |
  +-local/			-- for local configuration
  |
  +-README
  |
  +-scripts/			-- working scripts of the application
  | |
  | +-init.inc.php
  |
  +-var/			-- data directory

Now, I want to include the common php code underneath the lib directory, I merely add it as a submodule from the APP_ROOT directory:

  $ git submodule add   https://git.gitorious.org/common-php-code/common-php-code.git lib/commoncode

This clones the common code repository as a submodule in the earthquake project into the directory lib/commoncode. One thing I did in addition to this to make the common code scripts I'm going to be using "first-class" scripts is to link them into the lib directory:

  $ cd lib
  $ ln -s commoncode/class.Debug.php .
  $ ln -s commoncode/functions.inc.php .

And there we have it!

Cloning your project

Cloning your project (say to install it elsewhere to either work on or as a prodution system) takes a couple more steps.

After running the cloning process:

  $ git clone https://git.gitorious.org/earthquakwatch/earthquakwatch.git newdir

Initialize and update the submodules:

  $ cd newdir
  $ git submodule init
  $ git submodule update

Keeping submodules up to date

This is pretty straight-forward as well.

  $ cd APP_ROOT/lib/commoncode
  $ git fetch master