<?php
 
class Site
{
  var $_DB;
  var $_User;
 
  function Site()
  {
    $this->_DB = new DB();
    $this->_User = new User_Authentication();
  }
 
  function process()
  {
    // some database access is needed in this function
    $this->_DB->query();
  }
}
 
class DB
{
    // ... some class implementation goes here ...
}
 
class User_Authentication
{
  var $_local_DB;
 
  function User_Authentication()
  {
    // We reference to DB object in $SITE if exists,
    // otherwise we'll instantiate a new copy of DB locally
    if (isset($GLOBALS['mysite']->_DB))
      $this->_local_DB =& $GLOBALS['mysite']->_DB;
    else
      $this->_local_DB = new DB();
 
    // A test if the global var $mysite exists or not at this point
    echo isset($GLOBALS['mysite']) ? 'Var $mysite is there' : 'The global var $mysite does not exist !';
  }
 
  function process()
  {
    // some database access is needed in this function
    $this->_local_db->query();
  }
}
 
// Let's run the test ...
$mysite = new Site(); // this will be a global variable
?> 

<?php
// simple
$bar = isset($foo) ? $foo : '';
// complicated
$bar = isset($foo[$test['index']]) ? $foo[$test['index']] : '';
?>

<?php
// Use this to standardize things..
function isset_empty(&$input)
{
	return isset($input) ? $input : '';
}
?>

<?php
$bar = isset_empty($foo);  // $foo is undefined at this point
echo $bar;
$foo = array(1 => 'World', 2 => 'Cup');
print_r($foo);
$bar = isset_empty($foo[10]);
print_r($foo);
 
// The outpust result
//        <-- emptry string, expected
// Array ( [1] => World [2] => Cup )
// Array ( [1] => World [2] => Cup [10] => )
 
?>

-------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();
    }
}

// function declaration with default value
function foo($bar = 'default value here'){ ... };
// function declaration with passing by reference
function foo(&$bar){ ... };

// This won't work in PHP < 5
function foo(&$bar = 'default value here'){ ... };

<?
// function declaration
function foo($bar = 'default value here') { ... };
// function calling
foo(); // valid call
foo(&$var); // this, however is consider deprecated in PHP >= 5
?>

<?php
// Don't do this !
$value = urldecode($_GET['something']);
?>

<?php
$query = htmlspecialchars($_GET['query']);  
$query = str_replace('%2522', '', $query);  
$query = str_replace('%27', '', $query);  
$query = str_replace('%2527', '', $query);  
?>

<?php
// good 
str_replace('&amp;', '&', htmlspecialchars($_GET['redirect']);
// bad
htmlspecialchars(urldecode($_GET['redirect']));
?>

$name = 'Darren'; // This one's better
$name = "Darren";

echo '<a href="' . $url . '">' . $text . '</a>';
echo "<a href=\"$url\">$text</a>";
// You cannot do a \n with single quote
echo "\nThis is a new line\n";