Problema in una query

<?php
$servername = "localhost";
$username = "emanuelegisonna0505";
$password = "";
try {
   $conn = new PDO("mysql:host=$servername;dbname=myDB", $username, $password);
   $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
   echo "Connected successfully";
} catch(PDOException $e) {
   echo "Connection failed: " . $e->getMessage();
}
if ($conn->connect_error) {
   die("Connessione al database fallita: " . $conn->connect_error);
}
$continenti_query = "SELECT * FROM continenti";
$continenti_result = $conn->query($continenti_query);
$stati_query = "SELECT * FROM stati";
$stati_result = $conn->query($stati_query);
echo "<h1>Continenti</h1>";
echo "<ul>";
while ($row = $continenti_result->fetch_assoc()) {
   echo "<li>" . $row['nome_continente'] . "</li>";
}
echo "</ul>";
echo "<h1>Stati del Mondo</h1>";
echo "<table border='1'>";
echo "<tr><th>Nome Stato</th><th>Continente</th></tr>";
while ($row = $stati_result->fetch_assoc()) {
   echo "<tr><td>" . $row['nome_stato'] . "</td><td>" . $row['nome_continente'] . "</td></tr>";
}
echo "</table>";

$conn->close();
?>

perché quando apro la pagina mi da errore nella riga: "$continenti_result = $conn->query($continenti_query);"?