php如何判断用户名不存在

$username = $_POST['username'];

try{
    //连接数据库
    $pdo=new PDO('dsn', 'username', 'password');
} catch(\PDOException $e) {
    exit("连接错误：$e->getMessage()");
}
 
//查询
$sql = "select * from user where username=? limit 1";
//准备sql模板
$stmt = $pdo->prepare($sql);
//绑定参数
$stmt->bindValue(1, $usenname);
//执行预处理语句
$stmt->execute();
//推荐这种方式来获取查询结果
$data = $stmt->fetch(PDO::FETCH_ASSOC);
if (count($data) < 1) {
    echo "用户名不存在！";
}
//释放查询结果
$stmt = null;
//关闭连接
$pdo = null;