GLOBALS

The general consensus is that the use of globals is bad practise partly because they are difficult to debug and have excessive overhead. Globals are easy to override, and once you have done that debugging becomes almost impossible.

What ever problem you think you are solving by using globals you will find an easier, more logical, solution that does not use them.

While not directly addressing the "evil" of globals - this article is good reading to understand the thinking around the patterns that have been developed that specifically avoid globals

http://tutorials.jenkov.com/dependency-injection/dependency-injection-replacing-factory-patterns.html

If you are interested in make some variables available in your system, try another mechanism.
You can use some Class to get and set specific globals for your application, but do not use GLOBAL as a data store (class attributes instead).
If you declare some global variable more than once, nothing can alert you that your declaration is overwriting some variable in another side of your code.
If you create a class (or even a global function) you are in best position to ensure your code is not interfering with another code. For classes and functions, you are in control over your global resources.


Here is a basic example:

class App {

     var $db;
     var $anotherVar;


    public function connect(){
      //your connection code
      $this->db=$connection:
   }

   public function getDb(){
      if ($this->db===null) {
          $this->connect();
      }
     return $this->db;
  }

}

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And then you can access it in your code (after including the class file)

function getData($id){
    App::getDb()->query("...");
  ....
}

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Ok, so if I understand right I need to pass the reference of a database object, for example this wont work:-

<?php
class db {
     public function getDb(){
  		return "true";
  }
}

$dbConn = new db;

echo $dbConn->getDb();


class db2 {
     public function test(){
  		$dbConn->getDb();
  }
}

$ttt = new db2();
echo $ttt->test();

?>

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However if I passed the variable of the database object to a new object can I reference it that, something like:-

<?php
class db {
     public function getDb(){
  		return "true";
  }
}

class db2 {
	var $db2Conn;
	
     public function __construct($dbConnection) {
  		$db2Conn = $dbConnection;
  }
	public function test() {
		$this->db2Conn->getDb();
	}
}



$dbConn = new db;
echo $dbConn->getDb();

$ttt = new db2($dbConn);
echo $ttt->test();

?>

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Mostly - you need to add $this as below to be able to set the internal class variable.

public function __construct($dbConnection) {
   $this->db2Conn = $dbConnection; // Missing $this reference
}

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This is hyper-simplified, but maybe it will illustrate the essential moving parts of dependency injection, showing how the $mysqli object is not used as a global, but is injected into the function.  Injection into object methods is exactly the same as injection into inline functions.  You just name the thing you want to inject among the variables that are listed in the function definition, and use the thing inside the function or method.
http://iconoun.com/demo/mysqli_di_example.php

<?php // demo/mysqli_di_example.php

/**
 * Demonstrate Dependency Injection with a MySQLi Object
 *
 * Man Page References:
 * http://php.net/manual/en/functions.arguments.php#functions.arguments.type-declaration
 * http://php.net/manual/en/mysqli.overview.php
 * http://php.net/manual/en/class.mysqli.php
 * http://php.net/manual/en/class.mysqli-stmt.php
 * http://php.net/manual/en/class.mysqli-result.php
 * http://php.net/manual/en/class.mysqli-warning.php
 */

// RAISE THE ERROR REPORTING LEVEL TO THE HIGHEST POSSIBLE SETTING
ini_set('display_errors', TRUE);
error_reporting(E_ALL);
echo '<pre>';

// DATABASE CONNECTION AND SELECTION VARIABLES - GET THESE FROM YOUR HOSTING COMPANY
$db_host = "localhost"; // PROBABLY THIS IS OK
$db_name = "??";
$db_user = "??";
$db_word = "??";

// OPEN A CONNECTION TO THE DATA BASE SERVER AND SELECT THE DB
$mysqli = new mysqli($db_host, $db_user, $db_word, $db_name);

// DID THE CONNECT/SELECT WORK OR FAIL?
if ($mysqli->connect_errno)
{
    $err
    = "CONNECT FAIL: "
    . $mysqli->connect_errno
    . ' '
    . $mysqli->connect_error
    ;
    trigger_error($err, E_USER_ERROR);
}

// ACTIVATE THIS TO SHOW WHAT THE DB CONNECTION OBJECT LOOKS LIKE
// var_dump($mysqli);


// A FUNCTION THAT USES AN INJECTED DATABASE CONNECTION
function run_a_query($db, $query)
{
    if (!$res = $db->query($query))
	{
	    $err
	    = 'QUERY FAILURE:'
	    . ' ERRNO: '
	    . $mysqli->errno
	    . ' ERROR: '
	    . $mysqli->error
	    . ' QUERY: '
	    . $sql
	    ;
	    trigger_error($err, E_USER_ERROR);
	}
	$data = $res->fetch_object();
	return $data;
}


// USE THE FUNCTION BY INJECTING THE $mysqli OBJECT AND THE QUERY
$set = run_a_query($mysqli, 'SELECT 2+2 AS ANSWER');
var_dump($set);

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I'm marking this as a "Good Question" since it inspired this article.  Thanks for asking a question that a lot of others can benefit from!