js php xmlrequest upload pictures_PHP tutorial

I originally wanted to use a plug-in to upload images, but later I wrote a simple js to implement asynchronous image upload. Needless to say, the code is very simple

upload.php

if(isset($_FILES["myfile"]))
{
$ret = array();
$uploadDir = 'images'.DIRECTORY_SEPARATOR.date("Ymd").DIRECTORY_SEPARATOR;
$dir = dirname(__FILE__).DIRECTORY_SEPARATOR.$uploadDir;
file_exists($dir) || (mkdir($dir,0777,true) && chmod($dir,0777));
if(!is_array($_FILES["myfile"]["name"])) //single file
{
$fileName = time().uniqid().'.'.pathinfo($_FILES["myfile"]["name"])['extension'];
move_uploaded_file($_FILES["myfile"]["tmp_name"],$dir.$fileName);
$ret['file'] = DIRECTORY_SEPARATOR.$uploadDir.$fileName;
}
echo json_encode($ret);
}

?>

index.html

.txt{ height:28px; border:1px solid #cdcdcd; width:670px;}
.mybtn{ background-color:#FFF; line-height:14px;vertical-align:middle;border:1px solid #CDCDCD;height:30px; width:70px;} .file{ position:absolute; top:0; right:80px; height:24px; filter:alpha(opacity:0);opacity: 0;width:260px }