php: how to tell shell_exec to be done in background, and execute the next line

I have sample.php file as follows.

<?php
    shell_exec('export DISPLAY=:0.0 && xterm -hold -e "ls -al"');
<?

When i open it (localhost/~user/sample.php) in browser xterm opens and shows the ls -al command output.

but until i close the xterm the browser keep showing the loading thing rotating on the tab in firefox.

I dont want php to wait till i close xterm, instead after opeing the xterm, i want to convery that the page is complete.

I want the page to close after the xterm is opened.