PHP recursive query based on sql select

Question

I have 2 tables in a MariaDB

Table 1 structure:

ID (primary auto-increment) itemType(ENUM) and itemName(varchar)

The ENUM has 4 possibilities - "heading","section","text","citation".

Table 2 structure

ID (primary auto-increment) TID(int) parentID(int) and childID(int)

though it's possible I may need another column to perform the recursion.

Table 1 contains the text, and Table 2 the relationships of the text to each other.

The purpose of the query is to produce a nested JSON array/object in a tree structure.

example:[{itemType:"heading",sub[{itemType:"section",sub:[{itemType:"text"},{itemType:"text"},{itemType:"text"},{itemType:"text"}]},{itemType:"section",sub:[{itemType:"text"},{itemType:"text"}]} ]}]

so it can produce an HTML tree such as

• Heading 1
  • Section 1
    • Text 1
    • Text 2
    • Text 3
    • Text 4
  • Section 2
    • Text 2
    • Text 4
• Heading 2
  • Section 2
    • Text 1
    • Text 3
  • Section 3
    • Text 2
    • Text 4
    • Text 5
    • Text 6

Problems A heading can only exist once in the list Sections and text can appear many times in the whole list but only once within each heading Sections can be a child of any header so it's not "tied" Text can be a child of any section so it's not "tied"

So I start with

$data = $pdo->query("SELECT * FROM items WHERE itemType = 'header' ")->fetchAll();

The loop through $data

foreach( $data as $row ){writeMessage( $row['ID'],$row['ID']);}

Where writeMessage() is my recursive function

 function writeMessage($ID, $TID){
        global $pdo;    
        $stmt = $pdo->query("SELECT * FROM testing LEFT JOIN items ON items.ID = '$ID' WHERE parentID = '$ID' AND TID = '$TID' ");
         while ($row = $stmt->fetch()) {
            echo  $row['ID'].' '.$row['itemName'].' '.$row['childID'].'<br/>';
            writeMessage($row['childID'],$TID);
         }
     }

But I never get the right results.

Help and suggestions please, it's along time since I used PHP in this way