SQLite is an Embeddable SQL Database Engine [1]. Even if it has some limitations, there are at least already Subselects available. Earlier work [2] has been taken over by Wez and Tal. Since that night we have a woking PECL extension running on Windows too [3]; you might want to test my (unofficial) windows dll – provided without any warranty [4].
[1] http://www.hwaci.com/sw/sqlite
[2] http://sf.net/projects/sqlite-php
[3] http://pear.php.net/package-info.php?pacid=193
[4] http://circle.ch/php/files/php_sqlite.zip
