How to totally disable cache in WordPress?

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I have a plugin that somehow seems to be caching even though I have disabled cache in Firefox per these instructions:

https://support.mozilla.org/en-US/questions/1232148

I have also tried browsing in Private Window (like Chrome incognito).

In wp-config I have 

define('WP_CACHE', false);

In the plugin’s config file I have

if ( ! defined( 'LEARNDASH_TRANSIENTS_DISABLED' ) ) {
    define( 'LEARNDASH_TRANSIENTS_DISABLED', true );
}

No matter what I do, I notice it takes a couple minutes for edits in .php files to actually result in a change to the rendered broswer content.

Read that with wordpress server side cacheing can be quite tricky to fully disable.

Here is what I did as sanity check:

I edited a php file and included an echo statement with the word whattheheck. I refreshed the page, saw the text wqhattheheck then removed that echo statement and refreshed again and the page still displays whattheheck for several minutes after which point a refresh does not show the text.

I tried this after removing the echo statement:

grep -rwl “whattheheck” /var/www/mysite

which returned nothing. I also did a database dump and used

grep -n '/path/wordpress.sql' -e 'whattheheck'

which returned nothing. This is happening in all browsers, even say, Safari, where the first time I open the page in question it displays the text whattheheck. So there is absolutely no browser cache going on.

So, what we have here is a genuine mystery that begs the question:

If no text file within the site root contains the text that I previously added to a php file, and the wp_options table doesn’t contain the text (nor any other table), then how on earth is it that the page keeps showing this text for about 3 minutes after I remove the echo statement that produces the text?

Really curious. It is almost as if wordpress (or plugin) reads a file into memory and uses the memory stored version of the file for some time, which could explain what I am seeing. That is, I edit a file (in my case in PHPStorm), upload it, and wordpress or plugin uses previously loaded version of the file. But I have never read that this happens in wordpress.

thanks,

Brian

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