I would not change anything to the php.ini file, unless you know what you're doing.
The default settings are just fine to start learning php.
About your error
unknown function mysql_connect(); means that your php does not have mysql support .. installing the php-mysql package and then restart apache will resolve that issue.
create a file phpinfo.php and put this in it:
<?php phpinfo(); ?>
then open that file in your browser, you can see exactly what php supports at that moment. There should be a header called "Mysql"