A Useful Bit o' PHP Code, Set Right

16 June 2008

I came upon someone's ancient 'blog entry in which he or she attempted to present what would be a useful PHP function. Unfortunately, the code has a few bugs.

A dynamic webpage may seek data from various sources, including data passed by GET and POST methods (which is how web forms normally), persistent data in cookies (stored on the client but provided to the server with each visit), and persistent data on the server.

Towards that end, PHP maintains five associative arrays: $_GET, $_POST, $_COOKIE, $_REQUEST, and $_SESSION. ($_REQUEST combines the contents of $_GET, $_POST, and $_COOKIE.) To access a variable named user sent by POST method, one would refer to $_POST["user"], and so forth.

The 'blog entry in question may have been written before $_REQUEST was introduced; in any event, the author had two good ideas:
  1. Avoid errors resulting from trying to access variables that don't actually exist. If no variable user was passed by POST, then $_POST["user"] throws an error. To avoid that sort of thing, the author checks for the presence of the variable before attempting to access it.
  2. Combine the variables in $_SESSION, as well as those in $_GET, $_POST, and $_COOKIE. Indeed, session data is more analogous to cookie data than is cookie data to data transmitted by $_GET or by $_POST.
The problems with the actual code are these:
  • If the server is not maintaining session data, then the attempt to use $_SESSION will itself cause an error.
  • There is an attempt to get cookie data from the array $_SESSION.
  • In the aforementioned attempt, the array is treated as a function.
Here's a version of the code that fixes those problems:
function getvar($var_name)
  if (array_key_exists($var_name, $_GET) == TRUE) $ret_value = $_GET[$var_name];
  else if (array_key_exists($var_name, $_POST) == TRUE) $ret_value = $_POST[$var_name];
  else if (session_id() != "")
    if (array_key_exists($var_name, $_SESSION) == TRUE) $ret_value = $_SESSION[$var_name];
  else if (array_key_exists($var_name, $_COOKIE) == TRUE) $ret_value = $_COOKIE[$var_name];
  else $ret_value = ""; 
  return $ret_value;
PHP also provides analogous associative arrays for other global variables, but what unites the variable types of the five here is that they are commonly used in session-tracking — keeping data associated with a specific visitor as she moves through one's site. Possibly, getvar would be better named something else, if not distinguished by being made a member of some class of objects.

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