Wordpress debugging in actions/filters

I am developing a custom plugin in Wordpress. My plugin is dependent on other plugin (Event Calendar). I am calling certain hooks and filters of Event Calendar in my hooks but I am not able to debug or print_r my variables in that function. I tried setting WP_DEBUG to TRUE in wp-config.php, but that didn't helped. Currently, I am using mail function to get variable values. Is there any other way to print the variables?

via Chebli Mohamed