Error in single php line : Parse error: /wp-includes/option.php wp_cache_add

Question

The website randomly went down and I can't figure out how to correct the following error. I have tried toggling all plugins, the server was recently updated to PHP 7.4 from PHP 7.3 but no other website has had this issue.

My Setup:

PHP - PHP 7.4.11 (cli) (built: Oct  8 2020 17:32:43)

uname - Debian 4.19.146-1 (2020-09-17) x86_64 GNU/Linux

MySQL - mysql  Ver 15.1 Distrib 10.3.23-MariaDB, for debian-linux-gnu (x86_64) using readline 5.2

Error from the website:

Fatal error: Uncaught Error: Call to undefined function wp_cache_add() in /WEBSITE_PATH/wp-includes/option.php:258 Stack trace: #0 /WEBSITE_PATH/wp-includes/functions.php(1717): wp_load_alloptions() #1 /WEBSITE_PATH/wp-includes/load.php(664): is_blog_installed() #2 /WEBSITE_PATH/wp-settings.php(159): wp_not_installed() #3 /WEBSITE_PATH/wp-config.php(99): require_once('/home/swcsoldie...') #4 /WEBSITE_PATH/wp-load.php(37): require_once('/home/swcsoldie...') #5 /WEBSITE_PATH/wp-blog-header.php(13): require_once('/home/swcsoldie...') #6 /WEBSITE_PATH/index.php(22): require('/home/swcsoldie...') #7 {main} thrown in /WEBSITE_PATH/wp-includes/option.php on line 258

Fatal error: Uncaught Error: Call to undefined function wp_cache_add() in /WEBSITE_PATH/wp-includes/option.php:258 Stack trace: #0 /WEBSITE_PATH/wp-includes/option.php(116): wp_load_alloptions() #1 /WEBSITE_PATH/wp-includes/l10n.php(69): get_option() #2 /WEBSITE_PATH/wp-includes/l10n.php(137): get_locale() #3 /WEBSITE_PATH/wp-includes/l10n.php(828): determine_locale() #4 /WEBSITE_PATH/wp-includes/class-wp-fatal-error-handler.php(46): load_default_textdomain() #5 [internal function]: WP_Fatal_Error_Handler->handle() #6 {main} thrown in /WEBSITE_PATH/wp-includes/option.php on line 258

Code from Line 258:

wp_cache_add( 'alloptions', $alloptions, 'options' );

Answer 1

An undefined function error in PHP is fatal error, and means you're calling a function that doesn't exist (at least, at the point and scope it's being called at). Most often, I'd wager 9/10 times, this happens while you're developing and you mistype a function name, str_repace instead of str_replace , for instance.

With all of that being said, wp_cache_add() is a WordPress Core function defined in /wp-includes/cache.php , and is being called by a core file: /wp-includes/option.php on line 258.

Since these core files, and core functions are triggering a fatal "undefined function" error, it means your WordPress installation is failing somewhere. It possibly got corrupted during an update of some kind - it's infrequent, but it does happen for any number of reasons.

To fix this: try reinstalling WordPress, as it is absolutely compatible with PHP 7.4 .