OOP PDO Global Variables within Class - php

Question

I am looking into oop to enhance my web dev knowledge. At the moment i am having a bit of an issue. I have created a Database class that contains all the queries etc. ( fetch, count, etc. ) This just allows the queries or updates to take up less space in the other classes etc. The issue i am having is passing this class along and making it globally accessible. I have used
global $db;
within a class function but i read that it is bad practice to use that. I also do not want to pass the $db variable as a parameter if i did i would have to change a lot of my current classes and it would just be easier if i can make $db globally available in a "good" practice way.
I can provide my Database class if necessary it is just a simple class with the variable that initiates the connection through construct.
( Second Question )
I was also reading about the singleton instance function, before implementing i read that it was also considered bad practice. Is there something that should take its place?
( I decided to place the class below )
class Database {
private $host = 'localhost';
private $user = 'xxx';
private $pass = 'xxxx';
private $dbname = 'xxxxx';
private $dbh;
private $error;
public function __construct(){
// Set DSN
$dsn = 'mysql:host=' . $this->host . ';dbname=' . $this->dbname;
// Set options
$options = array(
PDO::ATTR_ERRMODE => PDO::ERRMODE_WARNING,
PDO::ATTR_EMULATE_PREPARES => false
);
// Create PDO Instance
$this->dbh = new PDO($dsn, $this->user, $this->pass, $options);
}
public function fetch($sql, $param = "") {
$this->stmt = $this->dbh->prepare($sql);
if (empty($param)) {
$this->stmt->execute();
} else {
$this->stmt->execute($param);
}
return $this->stmt->fetch();
}
}
$db = new Database();
An example of what i am attempting to accomplish is as follows
( User Profile Page )
class User {
function set_user($input) {
if (is_numeric($input)) {
$this->user = $input;
} else {
$user = $db->fetch("SELECT userid FROM users WHERE url=:url", array(':url' => $input));
$this->user = $user['userid'];
}
}
}

Answer 1

AN issue with your approach is not the global var, but that the DB connection is permanently open. It is not always good. This is kind of service object and it is not partiularily bad to have it globally visible.
If you don't want it global and want to solve the open connection issue, you can easily close the connection in the destructor, remove the global var and create/use/delete the DB wherever required.
Another approach would be not to use singleton, but so called service class. Make DB class not instanciable, but rather define service methods (all static) - open, close, execute, or whatever. Than the DB clients should not create DB object, but only access the static methods. This approach fits the reality very nice, as DB-access is seen as a service.