Such error happens in 1.7.x branch (I saw it in 1.7.2), when entering database parameters you’re seeing “PHP Extensions “0” must be loaded” instead of next step. This is internal Magento bug, you can correct it by finding file app/code/core/Mage/Install/etc/config.xml and replacing:
<extensions> <pdo_mysql/> </extensions>
with:
<extensions> <pdo_mysql>1</pdo_mysql> </extensions>