Fatal error: Uncaught Error: Call to undefined function wp_check_php_mysql_versions()

WordPress website was broken and inside the error_log file the following error was repeatedly printed: Fatal error: Uncaught Error: Call to undefined function wp_check_php_mysql_versions()

This fatal error indicates that WordPress was unable to check server configuration: PHP and MySQL versions.

In 99% of the time, the error is caused by missing or modified WP core files, in this particular case a restore process was running and still haven’t restored all WP core files.

If you encounter the same error then run WP Checksum and if any core files are detected as modified you should reinstall WordPress.

Verify WordPress Checksums

You can use the verify checksums commands in WP-CLI (the WordPress Command Line Interface).

wp core verify-checksums

This command downloads md5 checksums for the current version from WordPress.org, and compares those checksums against the currently installed files.

To check WordPress plugins, run:

wp plugin verify-checksums --all

Reinstall WP core

This WP-CLI command will download and install the current version of WordPress without the content folder (wp-content).

wp core download --force --skip-content

to reinstall a specific WordPress version use the version flag:

wp core download --force --skip-content --version=6.1

and if zou are using a specific locale, use the locale flag:

    wp core download --force --skip-content --locale=sr_RS --version=6.1