From Dolibarr Open Source ERP CRM Wiki
Following function allows to log informations in Dolibarr log file
The log level is related to setup defined in module Syslog (see lower).
Following function can be used to add error information on screen. Usage must be reserved in never reached code.
The error reporting module is integrated with other Dolibarr modules. It allows you to configure what level of errors you would like to be reported and where.
To get to the error reporting module configuration page, go to Home, Configuration, Modules and use the configuration icon of the last module on the screen: Syslog.
Of course, this module (Syslog) must be activated for the following to work.
The facility option allows you to use pre-set facilities to log your errors. Facilities are a system-level feature to tell your syslog operator where this error comes from, to judge priority and all that. You can find more about this inside the PHP documentation for the openlog() function which is used inside the dol_syslog() function: http://www.php.ne/openlog Really, you should leave it to LOG_USER unless you know exactly what you're doing.
The next option, file, allows you to choose a file in which you will store the logs. The path you will indicate there should be relative to the system root (/). If you live it as it is (default option DOL_DOCUMENT_ROOT/dolibarr.log), it will likely be opened in document directory. Of course, to do that, your web server must have write permissions on the directory you are using.
Generally, the file facility will work more easily because it doesn't depend on your server configuration (or only for the directory that needs to be writeable).
Finally, the level of reporting is how many types of errors you want to report. LOG_DEBUG(7) is the strongest option, reporting pretty much everything that calls a dol_syslog() function. LOG_EMERG(0) is the opposite, letting you see only the errors that would really block your system.
More information on page Module Syslog (developer).
Other rare system logs
Note that you can have also find some technical PHP/Apache errors in apache log file (whatever is status/setup of this module). But this occurs only if there is a technical problem in your installation: On a typical Linux install, such logs will be available somewhere in /var/log/apache/[dolibarr]-error.log. You can watch it live from the terminal by using:
tail -f /var/log/apache2/dolibarr-error.log