Small PHP script to set TYPO3_CONTEXT environment variable via php.ini setting for auto_prepend_file. Sets context depending on current subdomain (dev. or www.)

auto_prepend_file.php

<?php
$host = getenv('SERVER_NAME');
$patternDev = '/^dev\./';
$patternProduction = '/^www\./';

if (preg_match($patternDev, $host)) {
  putenv('TYPO3_CONTEXT=Development');
}
if (preg_match($patternProduction, $host)) {
  putenv('TYPO3_CONTEXT=Production');
}