Restart Apache from php error " Found 1 leftover httpd processes "

I want to restart apache from php.
I use this code

<?php
$output=shell_exec('/etc/init.d/httpd restart &');
echo $output;
exit;
?>

The output is this error

Stopping httpd: [FAILED] Found 1 leftover httpd processes. Killing. Starting httpd:

Does anyone know what's the problem?