oracle can't connect

<html>
<head><title>Oracle demo</title></head>
<body>
    <?php
    $conn=oci_connect("SYSTEM","123","localhost/orcl3");
    If (!$conn)
        echo 'Failed to connect to Oracle';
    else
        echo 'Succesfully connected with Oracle DB';

oci_close($conn);
?>

</body>
</html>