Vyskúšal som cez WHERE, a stále nič.

<?php
	session_start();
	$session_username = $_SESSION['user'];

	if ($session_username == "") {
		header("refresh=0; url=login.php");
	}

	function zisti_id() {
		$mysql_server = "localhost";
		$mysql_user = "root";
		$mysql_password = "None";
		$mysql_db = "gasdbgahs";
		$sqli = new mysqli($mysql_server, $mysql_user, $mysql_password, $mysql_db);
		if ($sqli->connect_errno) {
			printf("Connection failed: %s \n", $sqli->connect_error);
			exit();
		}
		$sqli->set_charset("utf8");

		$session_username = $_SESSION['user'];
		$sql = mysqli_query($sqli, "SELECT * FROM users WHERE id='$session_username' LIMIT 1");
		$zisti = mysqli_fetch_array($sql, MYSQLI_BOTH);

		echo $zisti["id"];
	}
?>