I have been working on a project on the Drupal 6 platform that requires the web application to recognize dynamically created sub-domains so the functionality of the database can be adjusted according to what sub-domain is in the current URL.
To get Drupal 6 to do this, I am creating a custom module. One of the things I have learned about creating a custom module in Drupal 6 is the implementation of the module’s “install” file.
If you create a module named say, my_module.module then the module’s install file name would be my_module.install and will reside in the directory named my_module located at….
my_site/sites/all/modules/my_module
Drupal 6 Module Install File Documentation
The .install file will be run by Drupal 6 the 1st time my_module has been enabled. The my_module.install file will then be used to run setup procedures as stipulated by your module. The most common task for the my_module.install file to perform is creating database tables and fields. There is no special syntax for the install file. It is simply a PHP file with a .install extension to properly identify it to the Drupal 6 platform.
There is special function used in the .install file called hook_schema(). Where hook would be replaced by the name of your module. For example, if the module you create is named my_module, then you would create a PHP file and name it my_module.install. Inside that file you will create a function named my_module_schema(). The my_module_schema() function is used to create arrays representingthe database tables you wish the module to create in the Drupal database. You can see how this works here.
Why do it this way?
Once you build the Drupal API Schema there is no more need for separate CREATE TABLE or ALTER TABLE statements on each database. As a module developer, you only need to create a schema structure and/or use the Drupal Schema API functions, and Drupal takes care of the rest. This greatly simplifies writing install and update functions.
This way of creating the module install files also allows for support in multiple database platforms.
According to Drupal’s Blog this also allows for “Several advanced capabilities, such as incremental database updates, a simple and consistent CRUD API, form scaffolding, simpler CCK and Views, schema and data validation, become much easier to implement in future enhancements.”