WP_CACHE results in Warning

[bjornenio]

Using the latest version with define( 'WP_CACHE', true ); in wp-config results in the following error: Warning: include(/wp-content/advanced-cache.php): failed to open stream: No such file or directory in /public_html/wp-settings.php on line 84

[zulfgani]

wp-settings.php is missing the && file_exists( WP_CONTENT_DIR . '/advanced-cache.php' ) check on line 82

[ginsterbusch]

@bjornenio To make this more understandable, start from original source, line 82 - the full snippet goes like this (1.1.2 is identicial to WP 4.9.13 in this case):

if ( WP_CACHE && apply_filters( 'enable_loading_advanced_cache_dropin', true ) ) {
    // For an advanced caching plugin to use. Uses a static drop-in because you would only want one.
    WP_DEBUG ? include( WP_CONTENT_DIR . '/advanced-cache.php' ) : @include( WP_CONTENT_DIR . '/advanced-cache.php' );

    // Re-initialize any hooks added manually by advanced-cache.php
    if ( $wp_filter ) {
        $wp_filter = WP_Hook::build_preinitialized_hooks( $wp_filter );
    }
}

So it states essentially if WP_CACHE (is true) AND the filter hook enable_loading_advanced_cache_dropin is true AS WELL, then it shoud look for the advanced-cache.php file in WP_CONTENT_DIR ..

So .. if this shows a warning, either WP_DEBUG is enabled, OR you're running PHP 8 (beta) already.

As advanced-cache.php is a drop-in replacement, the plugin (?) you're using failed to copy that into WP_CONTENT_DIR; one cause could be wrong file access permissions, including wrong OWNER (not just chmod 0755 or something similar).

cu, w0lf.

[bjornenio]

I understand that and I've changed the file now to include && file_exists( WP_CONTENT_DIR . '/advanced-cache.php' ) in the code. The warning goes away then.

Yes, I'm using wp_debug and wp_debug_display at all times to find errors. Using PHP 7.2 since later versions are not fully compatible with WP.

[bjornenio]

The reason for opening this "bug" was that I do not use a cache plugin, only define('WP_CACHE', true); in wp-config as I want the built in cache to work as intended.

I'm very new to Classic Press, so I don't know if there are things that do not work out of the box.

[ginsterbusch]

This is a bit off-topic, but: PHP 7.3 should be no issue at all. I'm currently in the finishing touches of a project with CP 1.1.2 as base, with quite a few common plugins (eg. ACF 5.8.8 Pro), with no issues whatsoever. If you need further help with that, just contact me directly my site ;)

[bjornenio]

I know, running 7.4.2 on my XAMPP local devs - but some things such as import of XML run into problems with later versions.

I was just surprised that WP_CACHE generated a problem.

[zulfgani]

Should define( 'WP_CACHE', true ); be generating the advanced-cache.php in the content folder? If not then how is the file generated if it's not present in the first place?

I've never used define( 'WP_CACHE', true ); so this is new to me. How was WP v4.9.x and by extension CP getting around it with out generating the error? Or was the error always there?

[bjornenio]

@zulfgani I do not know if adding it to wp-config should generate the file or not; meaning: I do not know what the intentions of CP are / were.

I use WP_CACHE when using WordPress without ever seeing an error or warning like this, which is what got me "confused".

I have not used any previous versions of CP and can therefore not say anything about if the error has always been there or not. I just discovered it and had to ask / raise a ticket.

[zulfgani]

I can confirm that the error is present in WP v4.9.x without the file_exists check in place so it's an inherited issue/bug.

[nylen]

define( 'WP_CACHE', true ); doesn't do anything by itself. You need an advanced caching drop-in in order to make this work: https://wordpress.org/support/article/editing-wp-config-php/#cache

This is a PHP file that you put in wp-content/advanced-cache.php. Usually it enables page caching or something related (not object caching like memcached, that is handled separately).

If you don't know what that is, then you should not set WP_CACHE. Many caching plugins will also manage both of these settings for you.

It seems like WP has included a change to avoid this warning in a 5.x release. In order to fix this we'd have to track down which changeset that was and then backport it.

I'm undecided on whether we should do this though. define( 'WP_CACHE', true ) without an advanced-cache.php file is actually an invalid configuration, and the current code looks like it will only provide this warning if WP_DEBUG is also enabled.

So this needs further investigation, and there is probably some relevant discussion on the WP ticket where this was changed (I haven't looked for it yet).

[kktsvetkov]

What about doing it in reverse ? E.g. set the value of WP_CACHE depending on whether advanced-cache.php exists, like this on line 82 in wp-settings.php

if (!defined('WP_CACHE'))
{
    define( 'WP_CACHE', file_exists( WP_CONTENT_DIR . '/advanced-cache.php' ) );
}

[ginsterbusch]

Because that line 82 in wp-settings.php reads quite differently, and that is not without reason:

if ( WP_CACHE && apply_filters( 'enable_loading_advanced_cache_dropin', true ) ) {
    // For an advanced caching plugin to use. Uses a static drop-in because you would only want one.
    WP_DEBUG ? include( WP_CONTENT_DIR . '/advanced-cache.php' ) : @include( WP_CONTENT_DIR . '/advanced-cache.php' );

    // Re-initialize any hooks added manually by advanced-cache.php
    if ( $wp_filter ) {
        $wp_filter = WP_Hook::build_preinitialized_hooks( $wp_filter );
    }
}

What it says here is: advanced-cache is ONLY enabled if BOTH WP_CACHE AND the filter enable_loading_advanced_cache_dropin return true. But out of some reason you DO NOT WANT THAT, eg. because you've already got a different caching solution running (ie. server-side), this leaves you still the option to auto-disable the drop-in replacement.

An auto-load like the one above WONT allow for this anymore. And as there are quite a big number of caching solutions, a few of them not just simple PHP scripts, I wouldn't dare breaking the current functionality. Yes, this would probably be a BREAKING change.

cu, w0lf.

[nylen]

And as there are quite a big number of caching solutions, a few of them not just simple PHP scripts, I wouldn't dare breaking the current functionality. Yes, this would probably be a BREAKING change.

This is correct.

[ClassyBot]

This issue has been mentioned on ClassicPress Forums. There might be relevant details there:

https://forums.classicpress.net/t/classicpress-1-2-0-rc1-release-notes/2479/23

[nylen]

define( 'WP_CACHE', true ); doesn't do anything by itself. You need an advanced caching drop-in in order to make this work: https://wordpress.org/support/article/editing-wp-config-php/#cache

define( 'WP_CACHE', true ) without an advanced-cache.php file is actually an invalid configuration

Closing because receiving a warning in this case is more correct behavior.