I'm trying my hands at my first plugin, nothing fancy just want to grab the user_id, create a folder for each user and let the user upload an image.

I know my code can't be completely wrong, because if I put the upload form into a html file and the file handling php code into a .php file on the server and then point the action= of the form to the php file it works. This is without the wordpress specific parts, of course. But apparently I fail to wrap that into a plugin so I can include the form through a shortcode in my pages, because nothing happens after I click the upload button.

Here's the code:

<?php
/**
 * Plugin Name: Upload Handler
 * Description: The plugin to handle uploads and such.
 */


/**
 *  THE UPLOAD HANDLING
 */


 function upload_handling(){
    //try to get the user ID from wp get_current_userid()
    $userid = get_current_user_id();

    //set upload directory equal to user ID under userfiles
    $uploadDir = "userfiles/" . $userid . "/";
    $target_dir = $uploadDir;
    $target_file = $target_dir . basename($_FILES["fileToUpload"]["name"]);
    $uploadOk = 1;
    $imageFileType = strtolower(pathinfo($target_file,PATHINFO_EXTENSION));


    //Check if directoy exists otherwise create it
    if (!file_exists($uploadDir)) {
        mkdir($uploadDir, 0777, true);
    }


    // Check if image file is a actual image or fake image
    if(isset($_POST["submit"])) {
    $check = getimagesize($_FILES["fileToUpload"]["tmp_name"]);
    if($check !== false) {
        echo "File is an image - " . $check["mime"] . ".";
        $uploadOk = 1;
    } else {
        echo "File is not an image.";
        $uploadOk = 0;
    }
    }

    // Check if file already exists
    if (file_exists($target_file)) {
    echo "Sorry, file already exists. Please rename it or choose another one.";
    $uploadOk = 0;
    }

    // Check file size
    if ($_FILES["fileToUpload"]["size"] > 750000000) {
    echo "Sorry, your file is too large.";
    $uploadOk = 0;
    }

    // Allow certain file formats only
    if($imageFileType != "jpg" && $imageFileType != "png" && $imageFileType != "jpeg" && $imageFileType != "bmp" && $imageFileType != "tiff"
    && $imageFileType != "gif" ) {
    echo "Sorry, only JPG, JPEG, PNG, BMP, TIFF & GIF files are allowed.";
    $uploadOk = 0;
    }

    // Check if $uploadOk is set to 0 by an error
    if ($uploadOk == 0) {
    echo "Sorry, your file was not uploaded.";
    // if everything is ok, try to upload file
    } else {
    if (move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $target_file)) {
        echo "The file ". basename( $_FILES["fileToUpload"]["name"]). " has been uploaded.";
    } else {
        echo "Sorry, there was an error uploading your file.";
    }
    }
}


/**
 * THE file UPLOAD FORM
 */

function upload_form() {
    echo '<form action="" method="post" enctype="multipart/form-data">';
    echo 'Select an image file:';
    echo '<input type="file" name="fileToUpload" id="fileToUpload">';
    echo '<input type="submit" value="Upload Image" name="submit">';
    echo '</form>';
}

if(isset($_POST['submit']))
{
   upload_handling();
} 


/**
 * THE SHORTCODE HANDLING
 */


//this is what happens when the shortcode is used
  function mh_shortcode() {
    //no idea what this does
   ob_start();
    //the upload button
 //   upload_file();
    //the name + file form
    upload_form();

    return ob_get_clean();
}


//Adding file handler shortcode
add_shortcode( 'file_handler', 'mh_shortcode' );

Any help would be appreciated.