mysqli类的对象无法转换为字符串

I am getting this error from the following code:

function updateRank($master_list, $event_id)
{
    $cnt_ml =  count($master_list);
    echo "count master list = $cnt_ml<br>";

    $b=1;

    for ($k=0; $k<$cnt_ml; $k++)
    {
        echo "master list element 1 - ".$master_list[1]."<br>";
        $foo = $master_list[$k];

        // Update each team in event_team table             
        $update = "UPDATE event_team 
                  SET pool_rank = $b 
                  WHERE event_id = $event_id
                  AND team_id = $foo";

        mysqli_query($conn, $update);// or die ('Could not run insert in event_team table');



        echo "|".$update."|<br>";   // Leave this line for debugging the sql query.
        $b++;
    }   
}

Specifically "$foo" is throwing the error. The "master_list" array is being passed correctly. When running the code, $cnt_ml returns "5" (which is absolutely correct). The line with:

echo "master list element 1 - ".$master_list[1]."<br>";

Returns "54" (which is correct).

Since the script is successfully able to read the array elements and post them, why is the script throwing the error when I include it in the UPDATE? It is able to see the array data but choking when trying to use the data in the UPDATE.

Thanks in advance!