-------site_management.php---------------
// The class of "everything", contains db connection
// and other authentication
class site_management
{
    var $DB_OBJECT;
    function site_management()
    {
        // Let's say MYSQL_DB is your class that contains
        // all functions to access/manipulate the database
        $this->DB_OBJECT = new MYSQL_DB();
    }
}
 
-------foo.php--------------------
// Some class that requires access to database
class foo
{
    // Internal database connection object
    var $_db_object;
 
    function foo()
    {
        // If global variable $SITE->DB_OBJECT exists, make reference
        // to it otherwise instantiate a new copy locally
        if (isset($GLOBALS['SITE']) && $GLOBALS['SITE']->DB_OBJECT)
            $this->_db_object =& $GLOBALS['SITE']->DB_OBJECT;
        else
            $this->_db_object = new MYSQL_DB();
    }
 
    function bar()
    {
        // You need to perform sql query here
        $this->_db_object->query($sql); // for example
        // ...
    }
}
 
------test.php-------------------
// The above two class is located in different files
// and below is also in a seperate script file
<?php
    $SITE = new site_management();
    $foo_obj = new foo();
    $foo_obj->bar();
?>

-------foo.php--------------------
// Some class that requires access to database
class foo
{
    // Internal database connection object
    var $_db_object;
 
    function foo()
    {
        // Instantiate a new copy locally
        $this->_db_object = new MYSQL_DB();
    }
}