How to do error logging in CodeIgniter (PHP)

I want error logging in PHP CodeIgniter. How do I enable error logging?
I have some questions:

1. What are all the steps to log an error?
   
   
2. How is an error log file created?
   
   
3. How to push the error message into log file (whenever an error occurs)?
   
   
4. How do you e-mail that error to an email address?
   

SEE: Below Link.

Codeigniter: Exception/Error Logging

What did you Try? What did you Get? What did you Expect?

Joined CodeIgniter Community 2009.  ( Skype: insitfx )

There is a built-in function to log errors called log_message Read about it HERE.

There is additional information about error handling on that page.
There is not any built-in mechanism to email logged errors though.

CodeIgniter has some error logging functions built-in. CodeIgniter error logging is mostly relatable to php error logging if you want to create it through code.

Make your /application/logs folder writable
In /application/config/config.php set
$config['log_threshold'] = 1;
or use a higher number, depending on how much detail you want in your logs
Use log_message('error', 'Some variable did not contain a value.');
To send an email you need to extend the core CI_Exceptions class method log_exceptions().