Mysqli doesn't work in PR4100?

I am trying to host my own web on PR4100 NAS MyCloud, i had already gone through all the symlink stuff, so that’s over and kicking.
Now i am facing another problem while coding my own simple php page.

I needed to access mySQL via php to retrieve data but it appears that i couldn’t do so without 500 Internal Server Error.
I’m not sure where went wrong because i couldn’t find the Error.log file that would show me my errors.

But as I remarked the codes line by line, i found out it started with $conn = new mysqli($servername, $username, $password, $dbname);

From what i can guess, its that the mysqli doesnt exist.
But how would WordPress and other apps connect to MySQL if it doesn’t work?

Is it possible that I need to activate it during the ?

Here’s a tiny code below and do let me know if I’ve missed something.

header("Access-Control-Allow-Origin: *");

//Setup and filter all inputs and variables
$u = isset($_POST['u']) ? $_POST['u'] : null;
$p = isset($_POST['p']) ? $_POST['p'] : null;

//Do works
if($method == 'POST')
	if(($u != null) && ($p != null))

		$servername = "localhost";
		$username = "test";
		$password = "1234";
		$dbname = "mydb";

		// Create connection
		$conn = new mysqli($servername, $username, $password, $dbname);
		// Check connection
		if ($conn->connect_error) {
			die("Connection failed: " . $conn->connect_error);

		$sql = "SELECT * FROM users";
		$result = $conn->query($sql);

		if ($result->num_rows > 0) {
			// output data of each row
			while($row = $result->fetch_assoc()) 
		} else {
			echo "0 results";


		//202 ACCEPTED
		//[X] 400 BAD REQUEST
	//[X] 400 BAD REQUEST

Many thanks in advance.