If you are referring to errors displayed on the webpage, you can enable it in php.ini (it is typically disabled for security on production systems). The value is called 'display_errors' (set to 1 to enable).
Alternatively, you can enable it on a per-script basis using the PHP error_reporting function.
In your PHP script add:
(Keep in mind that some hosts might not permit modifying certain values with ini_set. You should try the error_reporting function in preference.
If you are referring to errors in your server logs, ensure that error logging is enabled in your httpd.conf (for apache).