php array output with for loop dosent come as it should

I am trying to retrive data from database using FECH_ASSOC in order the output will took like this

last1 first1 last2 first2 & last3 first3

last4 first4 & last4 first4

this is the database i have:

ID  index  first    last
--------------------------
1     1    first1   last1  
2     1    first2   last2  
3     1    first3   last3  
4     2    first4   last4  
5     2    first5   last5  

Each output row shoud defined by the index value. as you can see in the output example I need the organ before the last one in any array to be separetad because i need to put the & before it.

this is the code i came up with:

//HERE I RETRIVE ALL THE DATA FROM THE DATABASE.
 $sql = "SELECT COUNT( * ) FROM table WHERE index= $index";
        $result = $dbh->prepare($sql);
        $result->execute();
        $number_of_rows = $result->fetchColumn();

//ASSOC DEFINITION
    $sth = $dbh->prepare("SELECT * FROM table WHERE index= $index);
    $sth->execute();
    $value = $sth->fetch(PDO::FETCH_ASSOC);

//THIS IS WHERE I CALL ALL THE ROWS BY THEIR VALUE
    $return = "";
                $fname                = $value['fname'];
                $lname                = $value['lname'];
                $numRow = $number_of_rows-2;   // THE ITEM BEFORE THE LAST ONE IN THE ARRAY
                $lastItem = $number_of_rows - 1; //LAST INDEX IN THE ARRAY
                    for($counter = 0; $counter <= $numRow; $counter++){
                    $value = $sth->fetch(PDO::FETCH_ASSOC);                 
                        $fname[$counter] = !empty($fname[$counter]) ? $fname[$counter] : '';

                    $return .= $lname. ', ' .   $fname;
                    }
                   $fname[$counter] = !empty($fname[$lastItem]) ? $fname[$lastItem] : '';

                    $return .= "&" . $fname. ', ' . $lname;
                    $return .= ' ';

            return $return; 

this is the output i get: last3 first1 last1 first1 & last2 first1

last5 first4 & last4 first4

I cant figure out what i am doing wrong. I tryied using foreach but i didnt manage to separete the last pair of values here.

I am stuck with this this for a few days. any help would be great.