Why installing Magento 1.7?

Some businesses consider this version because lots of plugins support it.

When is an error appearing on Magento 1.7 installation?

Always.

The error message is PHP Extensions “0” must be loaded

But Keep Calm, let’s solve this just modifying one line in config file.

Look near line 71 in file app/code/core/Mage/Install/etc/config.xml

You will find this

<extensions>
  <pdo_mysql/>
</extensions>

Change it to the following

<extensions>
  <pdo_mysql>1</pdo_mysql>
</extensions>

Source: PHP Extensions “0” must be loaded , while installing magento-1.7.0.1.tar.bz2 - Magento Forum