php-学生 求大神给段注释下面上传图片代码不知道怎么实现的

function getname($exname){
$dir = "upimages/";
$i=1;
if(!is_dir($dir)){
mkdir($dir,0777);
}

while(true){
if(!is_file($dir.$i.".".$exname)){
$name=$i.".".$exname;
break;
}
$i++;
}

return $dir.$name;
}

$exname=strtolower(substr($_FILES['upfile']['name'],(strrpos($_FILES['upfile']['name'],'.')+1)));
$uploadfile = getname($exname);

move_uploaded_file($_FILES['upfile']['tmp_name'], $uploadfile);
if(trim($_FILES['upfile']['name']!=""))
{
$uploadfile="admin/".$uploadfile;
}
else
{
$uploadfile="";
}