php的move_uploaded_file一直返回false是什么原因？

代码：

<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>
<?php
function 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;
    }


    。。。。。。。
    ?>