WHERE and AND in condition

[claro]

Greetings!

I have want to have a search query that will based on user preference.

$avail = $_POST['avail'];
$sec = $_POST['section'];

$query = "SELECT * ,b.b_Title as title, bac.id as bacid FROM tbl_book AS b
                      LEFT JOIN tbl_bdetail AS bd ON b.id = bd.b_Title
                      LEFT JOIN tbl_bacquisition AS bac ON bac.bd_Id = bd.id";

    if (count($avail)>0)
    {$query.="WHERE bac.ba_Status = '$avail'";}
        if (count($sec)>0)
    {$query.="WHERE b.section_Id = '$sec'";    } 

What I am trying here is that when the user select the $sec option, I want to add the WHERE clause value in my $query, when the user select the $avail option I want to add the WHERE clause values.

My problem is what if the user will select values among the two options? how can I make it happen. any help will do.