Something that took me a long time to figure out how to do is changing PHP configuration values with Virtualmin when mod_php is used. 

When you're using mod_php to process PHP scripts with Apache, the place to go in Virtualmin to change values for memory_limit, or max_execution_time is under the section called "Virtual Server Options". 

This "Virtual Server Options" section can be found by navigating the Virtualmin menus to Virtualmin -> Services -> Configure Website

Once you've followed that menu path, you should see a page with the heading that says "Virtual Server Options".  

From the "Virtual Server Options" page, look for the module titled "PHP". Click on it, and then you'll see the page where you can modify your PHP configuration for your site that's running on mod_php.

Once you've clicked on the "PHP" module, you'll see where you can modify or add your PHP Configuration values. Make sure that you save your configuration once you've finished making modifications.

Finally, remember to reload the configuration for your changed PHP configuration values to be applied. There's a little "refresh" icon at the top of the module page that has to be clicked before any of your changes will take effect.