Help with some PHP

function database($sql) {
  global $database;
  $connection=mysql_connect($database["host"],$database["username"],$database["password"]);
  if(!$connection) {
    die("Couldn't connect");
  }
  mysql_select_db($database["database"],$connection);
  $query=mysql_query($sql);
  return($query);
}

$SQL="SELECT $fields FROM `userData` WHERE `userID` = '$userID'";
echo $SQL;
$result=database($SQL);
while($row=mysql_fetch_array($result,MYSQL_ASSOC)){
  DoStuff();
}

 $database = new Database();
  $newUserID = $database->getUserID($userID);
 echo $newUserID;

config.php:
<?php
        //Save these 4 lines as config.php which we will include in our database.php file.
        //This is for security purposes.  Imagine a hacker getting into your database.php
        //file with the datbase password and username. Your users would be pissed!
        define('DB_HOST', 'localhost');
    define('DB_USER', 'username');
    define('DB_PASS', 'password');
    define('DB_NAME', 'db_name');
?>

database.php:
<?php

/* save this as database.php

then include the following in your original .php file that you had the code:
 include_once (database.php';
  $database = new Database();
  $userID = $database->getUserID();
  
*/
    include_once "includes/config.php";

        class Database
        {
                private $_userID;
                private $_db;
                
                private __construct()
                {
                        try {
                                $dsn = "mysql:host=".DB_HOST.";dbname=".DB_NAME;
                                %this->_db = new PDO($dsn, DB_USER, DB_PASS);
                            } catch (PDOException $e) {
                                echo 'Connection failed: ' . $e->getMessage();
                                exit;
                            }
                }
                
                public function getUserID($userID)
                {
                        $this->_userID = $userID;
                        $sql = "SELECT field 
                                        FROM userData 
                                        WHERE userID = :userID";
                                      
                      try
                      {
                          $stmt = $this->_db->prepare($sql);
                          $stmt->bindParam(':userID', $this->_userID, PDO::PARAM_INT); //if userID is string, change to PARAM_STR
                          $stmt->execute();
                         return $stmt->fetch(); //can also return $stmt->fetchColumn() and $stmt->fetchAll();
                      }
                      catch(PDOException $e) { echo $e->getMessage(); }
                }

                public function anotherDatabaseCall($userID)
                {
                        $this->_userID = $userID;
                        $sql = "SELECT * 
                                     FROM userData 
                                    WHERE userID = :userID";
                                      
                      try
                      {
                          $stmt = $this->_db->prepare($sql);
                          $stmt->bindParam(':userID', $this->_userID, PDO::PARAM_INT); //if userID is string, change to PARAM_STR
                          $stmt->execute();
                         return $stmt->fetchAll();
                      }
                      catch(PDOException $e) { echo $e->getMessage(); }
                }
        }

?>