How to create unlimited sub category in drop down list

How to create unlimited sub category in drop down list

Would you please guide me on how to create unlimited category and sub
category in a drop down list using php and mysql?
I was the sub category to have --- in front of them. For example, the
first sub category --- and the second level ----- and so on: I want to
achieve some thing like this:-
<select name="category">
<option value="1">Root</option>
<option value="3">- Sub </option>
<option value="4">- - Sub</option>
<option value="2">Root</option>
<option value="3">- Sub </option>
<option value="4">- - Sub</option>
</select>
Here is the code I got so far
function get_category($parentID = 0){
global $mysqli;
$html = '';
// Prepare the query
$stmt = $mysqli->prepare("SELECT CategoryID, Title
FROM category
WHERE ParentID =?");
// Execute the query
$stmt->bind_param('i', $parentID);
$stmt->execute();
// Store the result
$stmt->store_result();
// Bind the result
$stmt->bind_result($categoryID, $title);
while($stmt->fetch()){
$html .= "<option
value=\"$categoryID\">$title</option>";
$child = $mysqli->prepare("SELECT CategoryID
FROM category
WHERE ParentID =?");
// Execute the query
$child->bind_param('i', $categoryID);
$child->execute();
// Store the result
$child->store_result();
// Bind the result
$has_child = NULL;
$has_child = $child->num_rows;
if($has_child){
$html .= get_category($categoryID);
}
}
return $html;
}
echo '<select name="category">';
print(get_category());
echo '</select>';
Any help is appreciated