Function coding error concatenating quotes I think

If you’re evaluating php variables in a string(interpolating the variables directly in a string), it’s best to use double quotes, this way you don’t have to concatenante with dots.

function get_highest_bid( ) {
    global $wpdb;
    $postid = get_post_id();
    $table = $wpdb->prefix . "jwp_bids";
    $highest_bid = $wpdb->get_var(
        "Select max(bid_amt)
         FROM $table
         WHERE post_id ='$postid';"
         );
    echo $highest_bid;
}