PHP connection string to remote database ORA-12154

Question

I have 2 computers:
PC 1 - Here is where I installed the XAMPP.
PC 2 - Here is where my database which is Oracle 9i is installed.

I am using PHP 7 and already added PDO_OCI extension.

Here is my connection string:

define("DB_HOST", "192.168.10.30:1521");
define("DB_NAME", "BACKEND");
define("DB_USER", "sa");
define("DB_PASS", "sa_backend");

new PDO('oci:dbname='. DB_NAME . ';host='. DB_HOST .';', DB_USER, DB_PASS);

When I used this code I am getting this error:

Warning: Uncaught PDOException: SQLSTATE[42S02]: pdo_oci_handle_factory: ORA-12154: TNS:could not resolve the connect identifier specified (ext\pdo_oci\oci_driver.c:709)

UPDATE 1

$server         = "192.168.10.30";
$db_username    = "sa";
$db_password    = "sa_backend";
$service_name   = "backend";
$sid            = "backend";
$port           = 1521;
$dbtns          = "(DESCRIPTION = (ADDRESS = (PROTOCOL = TCP)(HOST = $server)(PORT = $port)) (CONNECT_DATA = (SERVICE_NAME = $service_name) (SERVER = SHARED) (SID = $sid)))";

$this->dbh = new PDO("oci:dbname=" . $dbtns . ";charset=utf8", $db_username, $db_password, array(
            PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
            PDO::ATTR_EMULATE_PREPARES => false,
            PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC));

I used this code I found on other stackoverflow link. Now I am getting this error

SQLSTATE[HY000]: pdo_oci_handle_factory: ORA-12520: TNS:listener could not find available handler for requested type of server (ext\pdo_oci\oci_driver.c:728)

I tried to updated the process to 200 even to 400 but the error is still the same.