This was a strange one after moving from PHP 7.4 to PHP 8.0. and PHP 8.1 I ran into an issue where the Advanced Custom Fields plugin function the_field() resulted in the following error.
Uncaught Error: Call to undefined function get_field() in /var/www/wp-content/themes/theme/header.php:146
The get_field() was in header.php and, as such, should be working, but I realized that perhaps the plugin isn’t loaded at this stage. So, I added the following to the top of the header.php file.
// Include acf.php include_once(ABSPATH.'wp-content/plugins/advanced-custom-fields-pro/acf.php');
And the error went away! The get_field was defined due to the Advanced Custom Fields plugin being included. I don’t know why this happened when switching to PHP 8.1
Jordan Trask
With 20 years of experience in technology spanning Linux, Data Center Infrastructure, and related services and applications, Jordan consults with small, medium, and large organizations tackling technology problems and needs. With 10 years of experience in WordPress, he’s created Managing WP as a means of dumping his brain on the web.
Jordan operates LMT Solutions providing Technology Consulting Services and We Power WP for WordPress Care Plans and Support.