How Does add_filter Work In WordPress - Developer Notes
Creating Filters - BEQDesigner
Fatal error: Call to undefined function add_filter() in phar:///usr/local/bin/wp/php/wp-cli.php(23) : eval()'d code on line 91 · Issue #1586 · wp-cli/wp-cli · GitHub
What is the difference between add_filter and apply_filters? | Labotrees
plugins - Passing a parameter to filter and action functions - WordPress Development Stack Exchange
Checkout on Front of Site Always On Even without products in cart? | WordPress.org
Call to undefined function add_filter() · Issue #3574 · wp-cli/wp-cli · GitHub