如果没有将递归设置为true，为什么mkdir无法工作？

我在摆弄我的代码，尝试编写一些代码，这些代码将通过POST从表单上载数据（包括图片），然后将创建一个包含相关子目录的目录来存储图像。

在编写代码时，我不断收到错误消息

Warning: mkdir(): No such file or directory in C:\Users\Admin\Desktop\UniServer\www\AddItem.php on line 94

但是，当我将mkdir的递归设置为true时，mkdir突然起作用，并且目录创建没有任何问题。

我的代码：

if(isset($_FILES['upload']['tmp_name']))
 {
 $numfile=count($_FILES['upload']['tmp_name']);
{
    for($i=0;$i<$numfile;$i++)
    {
        if(is_uploaded_file($_FILES['upload']['tmp_name'][$i]))
        {
            //Conditionals for uploaded file
            $foldername=$_SESSION['UserId'];
            $cat=$_POST['category'];
            $sub=$_POST['subcat'];
            $itemname=$_POST['itemname'];
            $allowed_filetypes=array('.jpg','.gif','.bmp','.png');
            $max_filesize = 2097152; // Maximum filesize in BYTES (currently 2.0MB).
            $upload_path = 'C:\Users\Admin\Desktop\UniServer\www\images\\'.$foldername.'\\'.$cat.'\\'.$sub.'\\'.$itemname.'\\'; // The place the files will be uploaded to.
            //Checks if Folder for User exists
            //If not, A folder for the user is created to store the user's images
            if(!file_exists($upload_path))
            {
                $upload_path=mkdir($upload_path,0644,true);<-- This is the line
            }

            $filename = $_FILES['upload']['name'][$i]; // Get the name of the file (including file extension).
            $ext = substr($filename, strpos($filename,'.'), strlen($filename)-1); // Get the extension from the filename.

            // Check if the filetype is allowed, if not DIE and inform the user.
            if(!in_array($ext,$allowed_filetypes))
            {
                die('The file you attempted to upload is not allowed.');
            }

            // Now check the filesize, if it is too large then DIE and inform the user.
            if(filesize($_FILES['upload']['tmp_name'][$i]) > $max_filesize)
            {
                die('The file you attempted to upload is too large.');
            }

            // Check if we can upload to the specified path, if not DIE and inform the user.
            if(!is_writable($upload_path))
            {
                $errormsg="Image Upload Failed.";
            }

            if(!move_uploaded_file($_FILES['upload']['tmp_name'][$i],"$upload_path" . $filename))
            {
                $errormsg= 'Your file upload was successful, view the file <a href="' . $upload_path . $filename . '" title="Your File">here</a>'; // It worked.
            }

        }
    }
}




 }
   else{echo"Upload failed";}

现在，我已经将递归设置为true，因此我的代码可以正常工作了，但是我仍然不知道为什么它可以正常工作，所以如果有人可以解释为什么我的代码确实可以正常工作，我将非常感激。

我最接近的是为什么mkdir在递归选项设置为true时失败？

虽然我不明白链接中所说的任何内容。

谢谢！