Rookie Progressbar Question

[isimpledesign]

0 down vote favorite

 

 

I know this is a rookie question but whats actually happening with a progress bar what is it actually monitoring.

 

I figure the only way to understand it is to get this answer.

 

when i run a piece off php code how can i get a progress bar to monitor it while it runs untill completion off the code??

 

most progressbars i have seen are file upload and download. is it monitoring the amount off requests the browser sends when upload the file so its monitoring the bytes that are being transfer?

 

Can someone please give me a better understanding off whats going on?

 

I just want a simple progress bar to monitor progress while my php code is running so say if i am running this simple code to backup a websites files and folders, how could i do this???

 

<?php if ( isset($_POST['backup']) ) {
      if(exec("cd {$_SERVER['DOCUMENT_ROOT']}/wp-content/plugins/s3bk/files;tar -cvpzf backup.tar ".get_option(    'isd-server')."")) { echo "done"; } 
}
<form id="backup" method='post' action=''>
<input type='submit' class="button" name='backup' value='backup'>

 

sorry for the rookie question just really want to get my head around whats actually happening???

 

Thanks