Change existing or add custom title or meta template variables

yoast_seo_title_change-variable.php

<?php
/********* DO NOT COPY THE PARTS ABOVE THIS LINE *********/
/* Remove Yoast SEO Change existing title or meta template variable
 * Credit: Moshe Harush
 * https://stackoverflow.com/questions/36281915/yoast-seo-how-to-create-custom-variables
 * Last Tested: Unknown
 */
 
 // define the wpseo_replacements callback
function filter_wpseo_replacements( $replacements ) {
    if( isset( $replacements['%%page%%'] ) ){
        $replacements['%%page%%'] = 'Page x of y';
    }
    return $replacements;
};
// Add filter
add_filter( 'wpseo_replacements', 'filter_wpseo_replacements', 10, 1 );

yoast_seo_title_custom-variable.php

<?php
/********* DO NOT COPY THE PARTS ABOVE THIS LINE *********/
/* Remove Yoast SEO Add custom title or meta template variables
 * Credit: Moshe Harush
 * https://stackoverflow.com/questions/36281915/yoast-seo-how-to-create-custom-variables
 * Last Tested: Nov 29 2018 using Yoast SEO 9.2.1 on WordPress 4.9.8
 *******
 * NOTE: The snippet preview in the backend will show the custom variable '%%myname%%'.
 * However, the source code of your site will show the output of the variable 'My name is Moses'.
 */

// define the custom replacement callback
function get_myname() {
    return 'My name is Moses';
}

// define the action for register yoast_variable replacments
function register_custom_yoast_variables() {
    wpseo_register_var_replacement( '%%myname%%', 'get_myname', 'advanced', 'some help text' );
}

// Add action
add_action('wpseo_register_extra_replacements', 'register_custom_yoast_variables');

I have created a quick article with screenshot and example variable current year and current month.

Read more https://maheshwaghmare.com/doc/yoast-seo-custom-template-variable/

@amboutwe it is possible to render the custom template variables in the snippet preview? thanks!

if someone needs to replace the custom variables also in the backend snippet preview, this is an example
https://gist.github.com/erikyo/53ec6a1b2264135dae95e866194a0a8a

How would I duplicate an existing template variable and modify it?

Or, in other words, how can I access $replacements['%%category_description%%'] in my replacement callback function?