CAT ID PARENT_ID
------+----------
   1      0
   2      0
   3      1
   4      1
   5      3
   6      2

Cat 1
Subcat1
SubCat2
SubSubcat1
Cat 2
Subcat1

<?php
require_once 'config.php';

?>
<?php header('Content-Type: text/html; charset=UTF-8'); ?>
<?php



$username = "qzd708";
$password = "aR689193";
$hostname = "qzd708.laclonadora.com";
$databasename = "qzd708";
$parent_id=0;
$salida=" ";
$mysqli = new mysqli($hostname , $username, $password, $databasename);
$ruta="";


//connect to mysql and select db
$conn = mysqli_connect($hostname , $username, $password, $databasename);

if( !empty($conn->connect_errno)) {die("Error " . mysqli_error($conn));}

echo "<select>";
echo "<option value='' disabled selected>--Seleccione Categoría--</option>";

//call the recursive function to print category listing
category_tree(0);

echo "</select>";
//
//echo gettheparent(168);
//Recursive php function

function category_tree($catid){

global $conn;
$sql = "select * from oc_category where parent_id ='".$catid."'";
$result = $conn->query($sql);

while($row = mysqli_fetch_object($result)):
$i = 0;
//$ruta=gettheparent(168);
if ($i == 0) {

    echo "<option value='". $row->category_id;echo "'>". gettheparent ($row->category_id) . getthename($row->category_id);
                        category_tree($row->category_id);
        echo "</option>";

    $i++;

}
 if ($i > 0) {
    //echo '</ul>';
        //$ruta=gettheparent($catid);



}

endwhile;

}

function category_tree_li($catid){
global $conn;

$sql = "select * from oc_category where parent_id ='".$catid."'";
$result = $conn->query($sql);

while($row = mysqli_fetch_object($result)):
$i = 0;
if ($i == 0) {
    echo '<ul>';
    echo '<li>' . $row->category_id;
    category_tree_li($row->category_id);
    echo '</li>';
    $i++;
}
 if ($i > 0) {
   echo '</ul>';

}

endwhile;

}

//category_tree_li(0);
//close the connection
mysqli_close($conn);

function getthename ($category_id){
    if ($category_id !=0)  {
    $username = "qzd708";
    $password = "aR689193";
    $hostname = "qzd708.laclonadora.com";
    $databasename = "qzd708";
    $mysqli = new mysqli($hostname , $username, $password, $databasename);
    if (mysqli_connect_errno()) {
        printf("Connect failed: %sn", mysqli_connect_error());
        exit();
    }
    $query = "SELECT name FROM oc_category_description WHERE category_id = '".$category_id."'";
    $result = $mysqli->query($query);
    while($row = $result->fetch_array())
    {
        $rows[] = $row;
    }
    foreach($rows as $row)
    {
       $salida.=" - ";
       $salida.= utf8_encode($row["name"]);
       return $salida;
    }
    $result->close();
    $mysqli->close();
}

    }