Elgg 1.x has been a pain to install since day one, back in October 2008 when the long-awaited 1.0 release came out a month late. By now it will likely be a snap to install on your typical vanilla shared LAMP host where they're used to all kinds of PHP/mySQL applications. (No troubles on Siteground, for example.) But of course Media Temple's gridserver (gs) is, as always, a special case of special pain.

Running PHP as an Apache module is stated as a requirement for Elgg that can be ignored. More complicated for (gs) users is that the (gs) has only just begun offering mySQL 5 in an opt-in beta program, and technically it is a requirement for Elgg. (Update: As of mid-February, new gs accounts have mySQL5.) You can run Elgg on mySQL 4, however, if you knock out its attempt to stop you. Here's how to do that and get going on the grid.

NB: I have not done this long enough with a variety of third-party plugins to say what will or will not break as a result of using mySQL 4. (2/15/09: So far, so good.)

Installing Elgg 1.2 on the (gs):

1. Upload the Elgg installation package where you want it to go. Don't fill out the web installer forms yet.
2. If you haven't already done so, create for Elgg a database and a database user with access to the database. Remember those names, as well as the database user's password as you will need them in step 6 below.
   
3. Make sure you have php5 turned on for the domain/folder where you are installing Elgg. The (gs) still defaults to php4 and Elgg smartly rejects it.
4. If you put the Elgg package's default .htaccess file into action (which is recommended), make sure you include a directive to run php5. The (gs) still uses php4 by default if you don't change this in Plesk. Take note that when you change the php version switches in Plesk it simply writes the directive AddHandler php5-script .php into the root .htaccess in the appropriate domain folder, so if you activate the default .htaccess file in the Elgg package later, you will revert to php4 and Elgg will not run.
5. If you put the Elgg package's default .htaccess file into action (which is recommended), remove the sections that check for php4 and return an error if they find it. Even with php5 running, on the (gs) these directives will result in php4 being detected, and Elgg will reject it.
   
6. Put the Elgg package's default settings.php into action. (It's in the "engines" folder) Put all your database information in the file with a text editor, and use your environment variable $_ENV{DATABASE_SERVER} instead of your internal database host name, although that will work too. (If you don't know what this means, read this.) Here is what your file should end up looking like, approximately, under the "standard configuration" section:
   

   // Database username
    $CONFIG->dbuser = 'db#####_YOURDBUSERNAME';
    
    // Database password
    $CONFIG->dbpass = 'YOURDBUSERPASSWORD';
   
    // Database name
    $CONFIG->dbname = 'db4####_YOURDBNAME';
    
    // Database server
    // (NOT localhost. Copy this next line exactly.)
    $CONFIG->dbhost = $_ENV{DATABASE_SERVER};
    
    // Database table prefix
    // If you're sharing a database with other applications, you will want to use this
    // to differentiate Elgg's tables. it can be anything.
    $CONFIG->dbprefix = 'elgg';
   
   

7. Remove the line in your root install.php file that checks for mySQL5, i.e.: validate_platform();
8. Now load up your Elgg folder in your browser, and fill out the forms with the requested information. By default the full, actual path for Elgg will be inserted into the textbox for yout site's URL. BE SURE TO DELETE THIS AND REPLACE IT WITH YOUR SITE URL. (Otherwise you'll get instant problems and have to make the correction in PHPmyAdmin or just drop your database tables and start over with the install form.)  
   
9. Your full, actual path should be detected and inserted in the textbox where it is requested. It will look like /nfs/c03/h04/mnt/####/domains/YOURDOMAIN.org/html/.
10. If you haven't done so already, create a folder called "files" one level above the folder you have installed Elgg in. The full, actual path to this folder needs to go in the last  install screen textbox, e.g. /nfs/c03/h04/mnt/####/domains/YOURDOMAIN.org/files/.
11. Complete the install process by creating the first, administrator account.
    
12. That's it.

NB: (mt)'s knowledgebase article on migrating sites and database applications from more conventional hosting to the grid is instructive about how the (gs) works by contrast to standard hosting.