Dropdown box changes header location PHP

I am working on a PHP script that will change the header of the page the user is currently on when a dropdown selection is made, the page will stay the same, but the information will alter. I am able to do this by manually changing the location in the URL bar, but so far have been unable to change the header automatically.

<select name = "rowno" onchange=header("Location: /Work/Log/new.php?rowno=$rowno")>

I am totally unsure of how to do this, so any help would be greatly appreciated, I have already set $rowno as a variable.