PDO prepared statement with optional parameters

Question

I'm kind of new with PDO and currently developing the API call that returns search results. How do I set a prepare statement if there are 2 optional parameters for the search query?

$app->get('/get/search', function () {
    $sql = 'SELECT * FROM user WHERE name LIKE :name AND city = :city AND gender = :gender';
    try {
        $stmt = cnn()->prepare($sql);
        $stmt->bindParam(':name', '%'.$_GET['name'].'%', PDO::PARAM_STR);
        $stmt->bindParam(':city', '%'.$_GET['city'].'%', PDO::PARAM_STR);
        $stmt->bindParam(':gender', $_GET['gender'], PDO::PARAM_INT);
        $stmt->execute();
        if($data = $stmt->fetchAll()) {
            echo json_encode($data);
        } else {
            echo json_encode(array('error' => 'no records found');
        }
    } catch(PDOException $e) {
        echo json_encode(array('error' => $e->getMessage()));
    }
}

The issue here, is that both $_GET['city'] and $_GET['gender'] are optional. If I try to run the code above, it will asume that any empty variable should match an empty value in the column as well; in the other hand, if I do something like this:

if($_GET['gender']) $stmt->bindParam(':gender', $_GET['gender'], PDO::PARAM_INT);

...it will return this error: "SQLSTATE[HY093]: Invalid parameter number: number of bound variables does not match number of tokens"

So, what's the solution if I want to keep the prepared sql statement for optional parameters? Thanks!

Update

This is the solution based on the accepted answer and some comments (by deceze and bill-karwin):

if($_GET['name']) $where[] = 'name LIKE :name';
if($_GET['city']) $where[] = 'city LIKE :city';
if(isset($_GET['gender'])) $where[] = 'gender = :gender';
if(count($where)) {
    $sql = 'SELECT * FROM user WHERE '.implode(' AND ',$where);
    $stmt = cnn()->prepare($sql);
    $name = '%'.$_GET['name'].'%';
    if($_GET['name']) $stmt->bindValue(':name', '%'.$_GET['name'].'%', PDO::PARAM_STR);
    $city = '%'.$_GET['city'].'%';
    if($_GET['city']) $stmt->bindParam(':city', $city, PDO::PARAM_STR);
    if(isset($_GET['gender'])) $stmt->bindParam(':gender', $_GET['gender'], PDO::PARAM_BOOL);
    $stmt->execute();
    if($data = $stmt->fetchAll()) {
        echo json_encode($data);
    }
}

Answer 1

Some good old dynamic SQL query cobbling-together...

$sql = sprintf('SELECT * FROM user WHERE name LIKE :name %s %s',
               !empty($_GET['city'])   ? 'AND city   = :city'   : null,
               !empty($_GET['gender']) ? 'AND gender = :gender' : null);

...

if (!empty($_GET['city'])) {
    $stmt->bindParam(':city', '%'.$_GET['city'].'%', PDO::PARAM_STR);
}

...

You can probably express this nicer and wrap it in helper functions etc. etc, but this is the basic idea.