Best way to integrate an existing PHP application into WordPress

A client of ours has a web application written in PHP. They have commissioned us to develop the front end for the website, but they want us to integrate the current PHP web application (a package tracking system) into the WordPress site (our choice). What’s the best practice to do this? Making the existing application into a plugin? Is that an approach?

Right now, the existing web application requires login and then establishes a session with the user. Can this web application co-exist within WordPress or do we just need to <IFRAME> it in or just link to it as a separate application?

Are there any best practices for this sort of thing?

Of course they could co-exist. The question is how much time and energy you want to put into it.

If it were me, I’d probably just link the logins and then add a link to the PHP application inside of WordPress. For example, if the user logs in inside WordPress it logs them into the application, then in the sidebar it has a submenu to the application with the user already logged in. Then if they logout of one make it so they log out of both.

This should be fairly easily done with sessions and wouldn’t take too much energy.

Creating a WordPress plugin would be nice, but it may be a lot more work than its value in my opinion at least.