Now, if your config file has an 'Solar_Example' group in it, those values will override any of the default values set by your class definition. Say your config file looks something like this:

<?php
$config = array();
// ...
$config['Solar_Example']['opt_3'] = 'dib';
// ...
return $config;

When you instantiate the Solar_Example object, the config file values will override the default values, leaving all others in place:

<?php
$example = Solar::factory('Solar_Example');
/**
 * 
 * The values of $example->_config are now:
 * 
 * 'opt_1' => 'foo'
 * 'opt_2' => 'bar'
 * 'opt_3' => 'dib' // from the config file
 */

Finally, if you specify a configuration array as the second parameter of Solar::factory(), those values override both the default values of the class definition and the Solar.config.php values.

<?php
$config = array('opt_2' => 'gir');
$example = Solar::factory('Solar_Example', $config);
/**
 * The values of $example->_config are now:
 * 
 * 'opt_1' => 'foo' // as defined by the class
 * 'opt_2' => 'gir' // from the Solar::factory() instantiation config
 * 'opt_3' => 'dib' // from the config file
 */

All of this is to say that the order of precedence for config values looks like this:

Note that values not changed remain the same, so if you leave one out, it's not overwritten to be null.