`includes/WebStart.php` allows for the possibility that `MW_CONFIG_FILE` has already been defined externally, but there is no mechanism in the code to facilitate it being predefined. My suggestion/request is to check the environment, see if a variable with such name is defined there, and if so pull its value in just as is already done with `IP` earlier in the same file.
The advantage of this feature would be that a virtual hosting web server could use the same MW code base to serve multiple different wikis simply by configuring this env var to point to the appropriate `LocalSettings.php` for each one.
Current workaround: In Apache, for each wiki I set:
```
SetEnv MW_CONFIG_FILE Path_to_this_wikis_LocalSettings.php
php_admin_value auto_prepend_file getMW_CONFIG_FILE_from_Env.php
```
Where `getMW_CONFIG_FILE_from_Env.php` contains just the one line:
```
define('MW_CONFIG_FILE', getenv( 'MW_CONFIG_FILE' ));
```
The present request would make only the one `SetEnv` customization line necessary for each host. I consider this to be a cleaner solution, at least for some situations, than other current options such as https://www.mediawiki.org/wiki/Manual:Wiki_family#Giant_switch_statement.