Change the search parameter from "letter" to something else

By default, the A-Z Entry Filter Extension uses the letter parameter to filter entries. For example, if you have a View at yoursite.com/view/example/ and you wanted to filter by entries that start with a, you would go to /view/example/?letter=a

How to modify the URL parameter used

The Extension has a gravityview_az_filter_parameter filter that allows you to change the parameter.

Instead of letter, you could use starts_with as the parameter, and the link would be /view/example/?starts_with=a instead of /view/example/?letter=a

Here's sample code that would do that:

add_filter( 'gravityview_az_filter_parameter', 'gravityview_az_filter_parameter_starts_with' );
/**
 * Change the A-Z Entry Filter parameter from `letter` to `starts_with`
 * @param  string $parameter Existing parameter
 * @return string            Modified parameter
 */
function gravityview_az_filter_parameter_starts_with( $parameter ) {
    return 'starts_with';
}

For reference, you can download and modify the plugin attached below.