[php] Warning: mysqli_select_db() expects exactly 2 parameters, 1 given in C:\

I'm doing a tutorial in which the author has not updated his content to reflect changes in the PHP documentation. Anyways, I need to know what is parameter is being asked of me to provide. I've checked that all things are in order, but I literally don't know what I'm supposed to provide. Here's what I have:

Connects.php

<?php
$connect_error = 'Sorry, we\'re experiencing connection issues.';
$con = mysqli_connect('localhost', 'root', 'PwdSQL5');
mysqli_select_db('phpcadet') or die($connect_error);
?>

And yet I get the error:

Edit: After figuring out to resovle the Connects.php issue, here's why I get when fixed it... more errors and here's my code. Remember I'm new to PHP and am following a poorly done tutorial.

Warning: mysqli_real_escape_string() expects parameter 1 to be mysqli, string given in C:\vhosts\phpcadet\core\functions\general.php on line 4

General.php

<?php
function sanitize($data)
{
return mysqli_real_escape_string($data, 'What goes here?');
}
?>

Then this: Warning: mysqli_query() expects parameter 1 to be mysqli, string given in C:\vhosts\phpcadet\core\functions\users.php on line 7

Users.php

<?php
require 'core/database/connects.php';

function user_exists($username)
{
  $username = sanitize($username);
  $query = mysqli_query($_POST['username'], "SELECT * FROM users");
  $row = mysqli_fetch_array($query);

  if($row['username']==$username)
  {
     echo "Welcome, $username!";
     return true;   
  }
  else echo "Please enter a valid username and password";
  return false;
}   

function user_active($username)
{
  $username = sanitize($username);
  $query = mysqli_query($_POST['username'], "SELECT * FROM users");
  $row = mysqli_fetch_array($query);

  if($row['username']==$username && $row['active']==1)
  {
     echo "Welcome, $username!";
     return true;   
  } 
  else echo "Your username and password could not be verified.";
  return false;
  }
 ?>

And finally this: Warning: mysqli_fetch_array() expects parameter 1 to be mysqli_result, null given in C:\vhosts\phpcadet\core\functions\users.php on line 8

See same code above

Some of my code is a compilation of other answers to solve issues with the lesson, and were found on Stack. Due to inconsistecy of them, it makes it hard for me to pick up what is going on... Coming from a different language, I not that familiar here, and just need the help, so it all clicks. Then I'll have "understanding" of what the manual is saying. Thanks.