/wp-content/cache/surge/ php files

I always like to know how plugins function, especially when it comes to something as important and as page caching. I tried browsing the /wp-content/cache/surge/ files, expecting to find static HTML files of page contents. Instead, I found .php files. When I try to open them, my editor gives me this message:

“The file is not displayed in the text editor because it is either binary or uses an unsupported text encoding.”

Are you able to help me understand a little bit more about these .php files, or direct me to some other reading about how Surge actually works?

Thank you!