Where is a bug in this script

<?php
session_start();
echo phpinfo();
mysqli_connect("localhost", "root", "", "my_base") or die(mysql_error());

$session = session_id();
$time = time();
$mysql = "SELECT * FROM 'user_online' WHERE session='$session'";
$result = mysql_query($mysql);
$count = mysql_num_rows($result);
if ($count == NULL) {
	mysql_query("INSERT INTO 'user_online' (session, time) VALUES ('$session','$time')");
}
else
	mysql_query("UPDATE 'user_online' SET time='$time' WHERE session = '$session'");
$count_user = mysql_query("SELECT * FROM 'user_online' ");
$count_user = mysql_num_rows($count_usert);
echo($count_user);
?>

Fatal error: Uncaught Error: Call to undefined function mysql_connect() in C:\xampp\htdocs\counter.php:7 Stack trace: #0 {main} thrown in C:\xampp\htdocs\counter.php on line 7