A little recursive function to kill a process and his childs.
it works fine for me and I don't have find something else to do it.
It's a mix of various scripts I've found.

<?php
    function killProcessAndChilds($pid,$signal) {
        exec("ps -ef| awk '\$3 == '$pid' { print  \$2 }'", $output, $ret);
        if($ret) return 'you need ps, grep, and awk';
        while(list(,$t) = each($output)) {
            if ( $t != $pid ) {
                killProcessAndChilds($t,$signal);
            }
        }
        //echo "killing ".$pid."\n";
        posix_kill($pid, 9);
    }
?>