13 questions about PHP configuration options

1. How PHP configurations can be changed?
2. What are major configuration values which we usually may want to change?
3. Mention the types of values which we can set in configuration values?
4. Can we use the ini_set function anywhere in our script?
5. Can we change and restore the configuration value to its original form from a script?
6. What are error reporting settings? How they affect the script?
7. What are session related settings in PHP?
8. What are database related settings?
9. How mails can be configured using configuration options?
10. How file based permissions can be managed using configuration options?
11. How can we secure the application using configuration options?
12. How we can disable the php functions in configuration?
13. What are the security risk we open by enabling register_globals?