关于全局global的有关问题

//写进日志
function write_logs($str='') {
global $num;
if($num>1){
    $num="ok";
}
    $fliepath_dir="test.txt";
    $str=$str."-".$num."\r\n";
    if($fp = @fopen($fliepath_dir, 'a')) {
        @flock($fp, 2);
        fwrite($fp, $str);
        fclose($fp);
        return true;
    }else{
        return false;
    }
}
$num=0;
$act=$_GET["act"];
if($act=="b"){
$num="2";
for ($index = 1; $index < 10; $index++) {
write_logs($index);
sleep(3);
}
}else{
    $num=0;
    echo "no".$num;
}