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Setup Domains and Sub-domains
It is recommended that you Create a Site before setting up the domain or sub-domain.
Step 1
Copy an existing Site's path.php and index.php (the ones from your main directory, not from the system directory) files into the directory from which you wish to access your new Site.
The directory you specify must be able to have files within it be able to access your main installation system directory. You should ensure you don't have any "open_basedir" or other restrictions in place which could prevent this.
If you want, you may rename the index.php file as described on the Renaming Main Site File page.
Step 2
Once you've copied the files, you must open path.php to edit it.
$system_path
First, you must ensure that the $system_path variable is set correctly within the file. The variable should contain a relative path from the current directory (in which the path.php file resides) to your site's system directory. For instance:
$system_path = "../system/";
Add Site Short Name Variable
Second, you must add the Site Short Name variable to the file:
$site_name = 'Site_Short_Name';
This will let ExpressionEngine know which Site's preferences to use for settings like main site index, the 404 page, etc... just like any ExpressionEngine installation.
Optional Variables
You may specify a PHP array containing "global variables" that will be available only to Templates accessed through that path.php file.
$global_vars = array(); // This array must be associative
For instance, let's say you want to define a weblog name and a Template Group name. You could use this:
$global_vars = array(
"my_weblog_name" => "news",
"my_template_group" => "display"
); // Last entry must not include the comma at the end
Within your Templates, you would then be able to use the {my_weblog_name} and {my_template_group} variables.