php的move_uploaded_file一直返回false

 <form action="?action=save&type=image&style=boolblue&language=zh-cn" method="post" name="myform" enctype="multipart/form-data"><INPUT TYPE="hidden" name="MAX_FILE_SIZE" value="width:100%" /> <input type='file' name="uploadfile" size='1' style="width:100%" /></form><?phpfunction DoSave(){    if($_FILES['uploadfile']['error'] != UPLOAD_ERR_OK){        switch((int)$_FILES['uploadfile']['error']){        case UPLOAD_ERR_NO_FILE:            OutScript("parent.UploadError('lang["ErrUploadInvalidFile"]')");            break;        case UPLOAD_ERR_FORM_SIZE:             OutScript("parent.UploadError('lang["ErrUploadSizeLimit"]+":".$GLOBALS["nAllowSize"]."KB"')");            break;        }        exit;       }    preg_match("/.([a-zA-Z0-9]{2,4})$/",$_FILES['uploadfile']['name'],$exts);    CheckValidExt($exts[1]);    $GLOBALS["sSaveFileName"] = GetRndFileName(strtolower($exts[1]));    $GLOBALS["sOriginalFileName"] = $_FILES['uploadfile']['name'];    $sFileName = str_replace('\','\\',realpath($GLOBALS["sUploadDir"]))."/";    $sFileName .= $GLOBALS["sSaveFileName"];    if(!move_uploaded_file($_FILES["uploadfile"]["tmp_name"],$sFileName)){        OutScript("parent.UploadError('Error')");到这就退出了        exit;    }    。。。。。。。    ?>