如何使用scandir列出文件夹和子文件夹中的所有文件，并将它们显示为select中的选项？

Question

I have a piece of code that I would like to modify to list all files in folders and sub-folders in the current path. I have found several possible solutions. I tried to implement them actually, but nothing seemed to work so I am not sure if they are actually working or not or it was just me implemented them wrong. Either way this is my current code:

<?php 
        $currentdir = 'C:/xampp/htdocs/test/'; //change to your directory
        $dir = opendir($currentdir);
        echo '<select name="workout">';
            $file = preg_grep('/^([^.])/', scandir($dir));
            while($file = readdir($dir))
            {
                if ($file != "." && $file != "..") {
                    echo "<option value='$file'>$file</option>";
            }           
                else {
                    continue;
            }
        }
        echo '</select>';
        closedir($dir); 
?>

Currently this code shows results:

$currentdir
   Option1
   Option2
   Option3
   SUB-FOLDER1
   SUB-FOLDER2

Can someone help me and show me how to write/rewrite the code using existing one to display files from folders and other sub-folders to look something like this:

$currentdir
  Option1
  Option2
  Option3
    SUB-FOLDER1
      Option1
      Option2
      Option3
    SUB-FOLDER2
      Option1
      Option2
      Option3

I became really desperate for a solution and thank you all in advance.

Answer 1

While the other answers are fine and correct, allow me to add my solution as well:

function dirToOptions($path = __DIR__, $level = 0) {
    $items = scandir($path);
    foreach($items as $item) {
        // ignore items strating with a dot (= hidden or nav)
        if (strpos($item, '.') === 0) {
            continue;
        }

        $fullPath = $path . DIRECTORY_SEPARATOR . $item;
        // add some whitespace to better mimic the file structure
        $item = str_repeat(' ', $level * 3) . $item;
        // file
        if (is_file($fullPath)) {
            echo "<option>$item</option>";
        }
        // dir
        else if (is_dir($fullPath)) {
            // immediatly close the optgroup to prevent (invalid) nested optgroups
            echo "<optgroup label='$item'></optgroup>";
            // recursive call to self to add the subitems
            dirToOptions($fullPath, $level + 1);
        }
    }

}

echo '<select>';
dirToOptions();
echo '</select>';

It uses a recursive call to fetch the subitems. Note that I added some whitespace before each item to better mimic the file structure. Also I closed the optgroup immediatly, to prevent ending up with nested optgroup elements, which is invalid HTML.