为什么我的函数返回值不可见？

im dealing with encapsulated categories here. I want to resolve them until i reached the top layer by recursive function:

    function resolve_kat($kat_id){

    ...credentials here...
    $db = new mysqli($host_name, $user_name, $password, $database, NULL, $socket);

    $sql_kat = "SELECT * FROM categories WHERE category=".$kat_id;
    $kat_res = $db->query($sql_kat);
    $kat = $kat_res->fetch_assoc();

    if($kat['upppercategory']==0){
        return $kat['category'];
    }
    else {
        resolve_kat($kat['upppercategory']);
    }

    $kat_res->close(); 
}

$passtome = resolve_kat(927);

echo $passtome;

It works fine with echo, but i want to pass the value to an other function. What did i wrong?

You look to be mising a return in your IF. Should be:

if($kat['upppercategory']==0){
   return $kat['category'];
}
else {
   return resolve_kat($kat['upppercategory']);
}

You can then pass it to another function.

$passtome = resolve_kat(927);

function otherFunction ($passtome) {
  #What you want to do...
}