<?php
   include("config.php");
   session_start();

   if($_SERVER["REQUEST_METHOD"] == "POST") {
      // username and password sent from form

      $myusername = mysqli_real_escape_string($db,$_POST['username']);
      $mypassword = mysqli_real_escape_string($db,$_POST['password']);

      $sql = "SELECT id FROM users WHERE username = '$myusername' and password = '$mypassword'";
      $result = mysqli_query($db,$sql);
      $row = mysqli_fetch_array($result,MYSQLI_ASSOC);
      $active = $row['active'];

      $count = mysqli_num_rows($result);

      // If result matched $myusername and $mypassword, table row must be 1 row

      if($count == 1) {
         session_register("myusername");
         $_SESSION['login_user'] = $myusername;

         header("location: welcome.php");
      }else {
         $error = "Your Login Name or Password is invalid";
      }
   }
?>

Warning: mysqli_fetch_array() expects parameter 1 to be mysqli_result, boolean given in /Applications/XAMPP/xamppfiles/htdocs/loginaspire/index.php on line 13

Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, boolean given in /Applications/XAMPP/xamppfiles/htdocs/loginaspire/index.php on line 16

$result = mysqli_query($db,$sql);
if(!empty($result) && $result !=null){
do rest of thing with $result
}else{
echo("Error description: " . mysqli_error($db));
}

<?php
session_start();
include("config.php");
error_reporting(E_ALL); // check all errors
ini_set('display_errors',1); // display all errors

if(isset($_POST['username']) && !empty($_POST['username']) && isset($_POST['password']) && !empty($_POST['password'])) { // check through values directly
   if($db){ // check if connection is made or not?
       $myusername = mysqli_real_escape_string($db,$_POST['username']);
       $mypassword = mysqli_real_escape_string($db,$_POST['password']);
       $sql = "SELECT id,active FROM users WHERE username = '$myusername' and password = '$mypassword' LIMIT 1"; // add column active too
      $result = mysqli_query($db,$sql);
      if($result){ // if query executed
        $row = mysqli_fetch_array($result,MYSQLI_ASSOC);
        $active = $row['active'];
        $count = mysqli_num_rows($result);
        // If result matched $myusername and $mypassword, table row must be 1 row
        if($count == 1) {
            session_register("myusername");
            $_SESSION['login_user'] = $myusername;
            header("location: welcome.php");
        }else {
            $error = "Your Login Name or Password is invalid";
        }
      }else{
          echo "Query error".mysqli_error($db);
      }
   }else{
       echo "connection error".mysqli_connect_error();
   }
}else{

    echo "Please fill form values";
}
?>

$count = mysqli_num_rows($result);
if($count > 0){
//Here your code
 $result = mysqli_query($db,$sql);
 $row = mysqli_fetch_array($result,MY`enter code here`SQLI_ASSOC);
 $active = $row['active'];
}else{
echo "No record found";
}