Trying to get class to instantiate ONCE (or, "why do I keep punching myself in the head?")

Question

Here's a very simple plugin, which I drop into mu_plugins in my multisite installation:

<?php
/*
  Plugin Name: Test
 */

class myclass {
    function __construct() {
        error_log("I just punched myself in the head.");
    }
}  // end of class definition

/* start of plugin functions */
function run_plugin() {
    static $counter = 0;
    if (0 === $counter++) 
        $i = new myclass();
}

/* start of execution code */
add_action('init','run_plugin');

OK - I log into WP admin, and I switch pages twice -- go to media library then themes. Counting the first landing on index.php there are three page renderings total.

Output of this plugin:

I just punched myself in the head.
I just punched myself in the head.
I just punched myself in the head.

So, given the static counter in the calling function - why is the constructor getting called three times? Or, is WP destroying my plugin and reloading it with every page load?

I'm very confused.

Answer 1

The init action will fire once on every page load. PHP doesn't maintain state between requests. If you want to run something once and never again forever unto eternity, you need to store something that persists between requests, like an option in the database, and check that before doing that one-time thing.

Answer 2

Unfortunately, this is not how PHP works. The PHP program is entirely executed (from start to finish) during a single request, i.e. when someone loads a page. Once you click any link (go to another page in your website) - everything is destroyed (every variable, class instance etc.) and the entire script starts from scratch.

In case of WordPress - the entire WordPress environment is loaded and executed every time someone requests a page. Every WordPress action (like the one you hooked into - 'init') is executed, all plugins and classes instantiated.

There is, of course, caching happening on the server side, but this is an advanced topic and probably not something you are interested in as a beginner.

As Milo said, if you want something to persist from page load to page load you have to store it in the database, most probably using transient API (if this is some kind of data other than, for example, a setting or post). If what you are doing is something that you would like to run once per user - you can store it in your user's browser as a cookie.