How to include seperate PHP file to class in callback function

Question

Your problem is with the path you’re using to include the file.

plugin_dir_path() just gets the directory of the given file/directory with a trailing slash. So if your code is here:

plugin/admin/views/key-analyzer-groups.php

Then dirname( __FILE__ ) will return

plugin/admin/views

So plugin_dir_path( dirname( __FILE__ ) ) will return

plugin/admin/

Which means that you are trying to include this path:

plugin/admin/admin/templates/groups.php

Which doesn’t exist.

If you want a reliable way to get the directory for your plugin to use throughout your code, then what you should do is define a function in your root plugin file that returns the path, then use that function throughout your code:

function key_analyzer_plugin_path() {
    return plugin_dir_path( __FILE__ );
}

Then use that function in your other files to include:

class Key_Analyzer_Groups {
    public static function init() {
        include_once key_analyzer_plugin_path() . 'admin/templates/groups.php';
    }
}

You could also tweak this approach to create a function for including files inside your plugin:

function key_analyzer_include( $file ) {
    $file = ltrim( $file, "https://wordpress.stackexchange.com/" );

    include plugin_dir_path( __FILE__ ) . $file;
}

Now you can use that function to include all your files by passing it a path relative to the main plugin file:

class Key_Analyzer_Groups {
    public static function init() {
        key_analyzer_include( 'admin/templates/groups.php' );
    }
}