PHP PDO语句不起作用

I'm having a problem with my PDO statement. I have an in active pipe tail -f /var/log/apache2/error.log watching for errors and nothing is coming in, I even piped my mysql.log file no errors there either.

• if i echo $_POST['sfl'] it will print an integer ie.(4)
• If I change the :sfl param to just a regular integer ie. "SELECT * FROM station WHERE SFL = '4'" it will query and pull the data fine.

I've removed error checks from the code for clarity

$sql = "SELECT * FROM station WHERE SFL = :sfl";
$stmt = $db->prepare($sql);
$stmt->bindParam(':sfl', $_POST['sfl']);
$stmt->execute();
while($result = $stmt->fetch(PDO::FETCH_ASSOC)){
    print_r($result);   
}

Question: Why isn't php binding my $_POST['sfl'] variable to my statement?