I received email from cronjob saying that there was PHP module already loaded:
PHP Warning: Module 'mysql' already loaded in Unknown on line 0
This warning happens when there is PHP extension loaded more than once. To fix this problem, check /etc/php5/apache2/php.ini file and the corresponding module configuration file in /etc/php5/conf.d/. For mysql module, I found it loaded 2 times in /etc/php5/apache2/php.ini and /etc/php5/conf.d/mysql.ini. Deactivate one module will settle the problem.
I also found that mysql.so extension is loaded 2 times on /etc/php5/cli/php.ini and /etc/php5/cli/conf.d/mysql.ini
I put ; (semi colon) in front of the line in /etc/php5/conf.d/mysql.ini