How to trigger a popup window/overlay in my module? I am stuck in my project. I am working on a module for account management page. When the user update their billing address, a service call will be triggered to send request to a third party API to get a response indicating if the address if valid or not. If valid then display the returned normalized address in a popup box or overlay, the user can choose if he want to use that address or not. My question is that how to trigger displaying this popup box in php code? 

Thanks!
Take a look at the Colorbox module.

You can find a few examples here.

Usage Example: 

&lt;a class="colorbox-load" href="http://example.com/?width=950&amp;height=500&amp;iframe=true"&gt;open URL in Colorbox (iframe)&lt;/a&gt;