Loading A PHP File/code Via Javascript.

As we all know, JavaScript is client side and php is server side, (the php
code is 'allowed' to do stuff on the server that JavaScript cannot).
The problem with php is that it timeout after a while, (and the user also
has no clue as to what is going on for a long time).

I need to run a script on the server that could take a very long time.

So what I was thinking is mixing both JavaScript and PHP
Something like,

<script>
var endvalue = 1000; /* some number that the server can calculate
quickly */
var i = 0
while (i<=endvalue)
{
/**
call a php file that will do some work
somefunction.php?someNumber=i
*/
}
</script>

That way the server does the work, while the client keeps it going.
Ideally I would also get a return value/string from the php script.