Description:
------------
Somewhat picking up on Bug #8577 here as I see some potentially legitimate uses for an "unset" function or some functional equivalent as a way to temporarily set a static value based on an expression. The replies in the last bug were a bit flippant on both sides so I hope to get a better resolution as I see this as a potential step towards another degree of untapped flexibility and ability to minimize code.

The following function definitions are not valid in PHP:

function connectFunct($user_name, $password, $server = getHostName(), $port=3306) {
   //some code
}
function connectVar($user_name, $password, $server = $_SERVER["SERVER_NAME"], $port=3306) {
   //some code
}


at present the only workaround is to have a "flag" value and reassign the variable as needed like so:

function connectFunct($user_name, $password, $server = null, $port=3306) {
   if ($server === null) {
     $server = getHostName();
   }
   //some code
}
function connectVar($user_name, $password, $server = null, $port=3306) {
   if ($server === null) {
     $server = $_SERVER["SERVER_NAME"];
   }
   //some code
}


However the need may easily arise where the "flag" value chosen becomes a value that triggers some other behavior (e.g. null should be an erroneous value if passed). If a constant could be redefined or undefined to more accurately reflect the current state of the value of function call or variable it would allow for a less "hackish" workaround.

Example using "undefine()":

if (defined('CONNECT_SERVER')) {
  $prev_connect_val = CONNECT_SERVER;
  undefine('CONNECT_SERVER');
}
define ('CONNECT_SERVER', getHostName());
function connectFunct($user_name, $password, $server = CONNECT_SERVER, $port=3306) {
   //some code
}
undefine('CONNECT_SERVER');
if (is_set($prev_connect_val)) {
  define ('CONNECT_SERVER', $prev_connect_val);
}


Alternately, you could have another class of constants which are designed for the purpose of temporary value storage (i.e. "semi-constants" mentioned in summary):

Example using "semi-constant" function define_temp():

define_temp ('CONNECT_SERVER', getHostName());
function connectFunct($user_name, $password, $server = CONNECT_SERVER, $port=3306) {
   //some code
}
undefine_temp('CONNECT_SERVER'); //may not be necessary depending on implementation of "semi-constants"