First of all, you'll need to make PHP to display "progress", probably you'll need to override ob_start() or something like that, and find a format that let you append new progress on the response on the fly.
I guess you already have an URL on your Wordpress setup that triggers this export. Let's call it {url}/export.
Wordpress already has jQuery by default included. So'll you need to call that URL using jQuery $.post and then, accordingly to the response, update your progress bar.
There is nothing specifically about Wordpress on this, besides the fact that you need to setup your own URL on Wordpress to do this, and then include your own JS after jQuery. That's all.
If you find this too-complicated, a quick-hack is to create a page on WP Admin called Export Tool, and then on your theme create page-export-tool.php. That .php will be called when visited that Export Tool page.
I guess you already have an URL on your Wordpress setup that triggers this export. Let's call it {url}/export.
Wordpress already has jQuery by default included. So'll you need to call that URL using jQuery $.post and then, accordingly to the response, update your progress bar.
There is nothing specifically about Wordpress on this, besides the fact that you need to setup your own URL on Wordpress to do this, and then include your own JS after jQuery. That's all.
If you find this too-complicated, a quick-hack is to create a page on WP Admin called Export Tool, and then on your theme create page-export-tool.php. That .php will be called when visited that Export Tool page.