virtual.prepend.php

<?php
// add this to your php.ini
// auto_prepend_file = /path/to/virtual.prepend.php

$http_host = explode('.',$_SERVER['HTTP_HOST']);
$__mod_vhost_alias_fix_doc_root = $_SERVER['DOCUMENT_ROOT'] . 
        DIRECTORY_SEPARATOR . $http_host[0] .
        DIRECTORY_SEPARATOR . 'public';

if (is_dir($__mod_vhost_alias_fix_doc_root)) {
    $_SERVER['DOCUMENT_ROOT'] = $__mod_vhost_alias_fix_doc_root;
}