Can PHP constants' values be changed per HTTP request? - php

Question

I'd like to know from a design POV whether changing the value of a constant per HTTP request is discouraged or perfectly ok.
I have a few constants that are defined at the start of a PHP script and they're used to figure out a particular context (what is the user trying to do at the time). The constants are never changed throughout the lifecycle of the script so they conform with the rules of constants and they work well. However, the values of these constants depends on what the user's doing. I'm wondering whether this is discouraged or perfectly acceptable in PHP.
<?php
// This function is only run once per HTTP request at the start
function new_paper() {
define('NEW_PAPER', 1);
define('NEW_VERSION', 0);
}
// This function is also only run once per HTTP request at the start
function new_paper_version() {
define('NEW_PAPER', 0);
define('NEW_VERSION', 1);
}
// This function is subsequently called by both functions above
function a_handler_of_sorts() {
if (NEW_PAPER) {
// Do something if it's a new paper
}
elseif (NEW_VERSION) {
// Do something if it's a new version
}
else {
}
}
In no circumstances are both new_paper() and new_paper_version() run in the same HTTP request.

Answer 1

A typical use case would be something like:
define('DEBUG', !empty($_GET['debug']));
if (DEBUG) echo 'some debugging statement';
Obviously don't do this only based on a query parameter, especially not in production, but you get the idea.
So, yes, setting constant values based on the request is fine. Whether this is the best thing to do in your particular case is questionable and I don't know. I'd really reserve it for "meta" values like debug flags, not for values which are essentially function parameters, inputs for your business logic. Do this sparingly.
"Constant values" which influence how every script works independently of the request would be something like config files or environment variables, e.g. containing database access credentials and such.

Answer 2

Constants should not change during the request but as you said yours don't so I think you're OK there.
They represent fixed things such as how many DB connections are allowed, the name of the application etc. If you're trying to use them to store the state of your application then you could consider doing something like:
<?php
define('STATE_PAPER', 0);
define('STATE_VERSION', 1);
define('STATE_INVALID', 2);
$applicationState = null;
if (someCheckForPaper() === true) {
$applicationState = NEW_PAPER;
} else if (someCheckForVersion() === true) {
$applicationState = NEW_VERSION;
} else {
$applicationState = STATE_INVALID;
}
// Save $applicationState somewhere... maybe session?
// Somewhere else
if ($applicationState === STATE_PAPER) {
...
}