Global variable not updating inside static Class method in PHP

Question

I have a problem with global variables in PHP. My problem is that a global variable that I change inside a static class method isn't updating outside the method.

I've included the code:

test.php

define( 'APP_ID', 'TESTING' );
$_APP = array( 'test' => 'test value' );
include ('appsettings.class.php');
AppSettings::initApplication();

appsettings.class.php

class AppSettings
{
  public static function initApplication()
  {
    global $_APP;
    session_start();

    // Some code here for your initializtions
    self::initAppEngine();
echo '<pre>Inside initApplication: '; print_r($_APP);
echo '<pre>Directly printing the session variable: '; print_r($_SESSION[APP_ID] );
  }

  private static function initAppEngine()
  {
    global $_APP;

    if( isset($_SESSION[APP_ID]) )
    {
      $_APP = &$_SESSION[APP_ID];
    }
    else
    {
      $_SESSION[APP_ID] = array( 'abcd' => 'hello', 'APP_ID' => APP_ID );
      $_APP = &$_SESSION[APP_ID];
die("Refresh the page");
    }

    if ( !isset( $_APP['uid'] ) )
      $_APP['uid'] = 0;

echo '<pre>Inside initAppEngine: '; print_r($_APP);
  }
}

The old value of $_APP is coming instead of the new one inside initApplication. Can anyone point out what I'm doing wrong?

Thanks in advance,

Answer 1

This is quite interesting. First of all, note that it seems to have nothing to do with static methods:

$_SESSION['test'] = array("test value from superglobal");
$_APP = array('test' => "test value directly assigned");

class AppSettings
{
  public static function initApplication()
  {
    global $_APP;
    $_APP = &$_SESSION['test'];
    echo '<pre>Inside initApplication: '; print_r($_APP);
  }

  public function initApplicationNonStatic()
  {
    global $_APP;
    $_APP = &$_SESSION['test'];
    echo '<pre>Inside initApplicationNonStatic: '; print_r($_APP);
  }
}

echo '<pre>Before calling initApplication: '; print_r($_APP);
AppSettings::initApplication();
echo '<pre>After calling initApplication: '; print_r($_APP);
echo '<pre>Before calling initApplicationNonStatic: '; print_r($_APP);
$appSettings = new AppSettings();
$appSettings->initApplicationNonStatic();
echo '<pre>After calling initApplicationNonStatic: '; print_r($_APP);

Result:

Before calling initApplication: Array
(
    [test] => test value directly assigned
)
Inside initApplication: Array
(
    [0] => test value from superglobal
)
After calling initApplication: Array
(
    [test] => test value directly assigned
)
Before calling initApplicationNonStatic: Array
(
    [test] => test value directly assigned
)
Inside initApplicationNonStatic: Array
(
    [0] => test value from superglobal
)
After calling initApplicationNonStatic: Array
(
    [test] => test value directly assigned
)

But this works:

$_SESSION['test'] = array("test value from superglobal");
$_APP = array('test' => "test value directly assigned");

class AppSettings2
{
  public function initApplicationNonStatic()
  {
    $GLOBALS['_APP'] = &$_SESSION['test']; // by reference
    echo '<pre>Inside initApplicationNonStatic: '; print_r($GLOBALS['_APP']);
  }
}

echo '<pre>Before calling initApplicationNonStatic: '; print_r($_APP);
$appSettings2 = new AppSettings2();
$appSettings2->initApplicationNonStatic();
echo '<pre>After calling initApplicationNonStatic: '; print_r($_APP);
$_SESSION['test'] = array("test value from superglobal altered");
echo '<pre>After altering superglobal: '; print_r($_APP);

Result:

Before calling initApplicationNonStatic: Array
(
    [test] => test value directly assigned
)
Inside initApplicationNonStatic: Array
(
    [0] => test value from superglobal
)
After calling initApplicationNonStatic: Array
(
    [0] => test value from superglobal
)
After altering superglobal: Array
(
    [0] => test value from superglobal altered
)

And this works, too:

$_SESSION['test'] = array("test value from superglobal");
$_APP = array('test' => "test value directly assigned");

class AppSettings2
{
  public function initApplicationNonStatic()
  {
    global $_APP;
    $_APP = $_SESSION['test']; // by value
    echo '<pre>Inside initApplicationNonStatic: '; print_r($_APP);
  }
}

echo '<pre>Before calling initApplicationNonStatic: '; print_r($_APP);
$appSettings2 = new AppSettings2();
$appSettings2->initApplicationNonStatic();
echo '<pre>After calling initApplicationNonStatic: '; print_r($_APP);
$_SESSION['test'] = array("test value from superglobal altered");
echo '<pre>After altering superglobal: '; print_r($_APP);

Result:

Before calling initApplicationNonStatic: Array
(
    [test] => test value directly assigned
)
Inside initApplicationNonStatic: Array
(
    [0] => test value from superglobal
)
After calling initApplicationNonStatic: Array
(
    [0] => test value from superglobal
)
After altering superglobal: Array
(
    [0] => test value from superglobal // expected, since assigned by value
)

So, it seems that whenever you want to assign a reference to a global variable inside a function or method, you have to use the $GLOBALS['_APP'] syntax and you cannot use global $_APP. If you don't need the assignment by reference, $GLOBALS['_APP'] and global $_APP behave the same.

I'm not exactly sure why this is so; some pages refer to the equivalence of these two constructs:

global $example;
$example =& $GLOBALS['example'];

This might lead to the right track; however, I hope that you can troubleshoot your problem with my answer.