致命错误：在非对象上调用成员函数execute（）

I have a problem with the execute(): Fatal error: Call to a member function execute() on a non-object I seem to have used it correctly, I looked up other questions here on StackOverflow and I think it is just right.

  $db = dbConnect();
  $query = "SELECT * FROM 'users' WHERE 'username' = '".$username."'";
  $stmt = $db->prepare($query);
  $stmt->execute();

Where dbConnect() is the function:

  function dbConnect() {
      $db = mysqli_connect("localhost", "root", "example", "dbexample")
          or die("Errore nella connessione al db: " . mysqli_connect_error());
      return $db;
  }

Thanks

Please use this:

$query = "SELECT * FROM `users` WHERE 'username' = '".$username."'";

instead of:

$query = "SELECT * FROM 'users' WHERE 'username' = '".$username."'";

Try this

<?php
$db = mysqli_connect("localhost", "root", "example", "dbexample")
          or die("Errore nella connessione al db: " . mysqli_connect_error());
      $stmt = $db->prepare("SELECT * FROM users where username =".$username);
      $stmt->execute();
?>