How to do error logging in CodeIgniter (PHP)

CodeIgniter has some error logging functions built in.

• 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(). You can do this yourself or use this. More info on extending the core here

See http://ellislab.com/codeigniter/user-guide/general/errors.html