Ability to use envvars in Magento config

Original module: https://github.com/rossigee/magento-config-envvars which used $_SERVER vars which did not work for us using HHVM.

In our development workflow, our webservers provide environment configuration information to our applications via environment variables (i.e. MAGE_DB_HOST, MAGE_DB_USER etc). This doesn't sit well with Magento's XML based configuration approach.

This little hack allows us to use '$MAGE_DB_HOST' and any other environment variables in our XML configuration files.

You can also specify a default value in case the envvar is missing or empty by appending it with double pipe symbols (e.g. '$MAGE_DB_HOST||localhost').

Usage

Example part of our local.xml...

...
            <default_setup>
                <connection>
                    <host>$MAGE_DB_HOST</host>
                    <username>$MAGE_DB_USER</username>
                    <password>$MAGE_DB_PASS</password>
                    <dbname>$MAGE_DB_NAME</dbname>
                    <initStatements><![CDATA[SET NAMES utf8]]></initStatements>
                    <model><![CDATA[mysql4]]></model>
                    <type><![CDATA[pdo_mysql]]></type>
                    <pdoType></pdoType>
                    <active>1</active>
                </connection>

...