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<?php
/**
 * Plugin Name: Adsense after more-Tag
 * Description: Add content of var $extra_more after more-Tag
 * Version:     0.0.1
 * Author:      Frank Bültge
 * Author URI:  http://bueltge.de/
 */
 
! defined( 'ABSPATH' ) and exit;

add_filter( 'the_content_more_link', 'custom_more_link', 9999 );
function custom_more_link( $more_link ) {
    
    $extra_more = 'Adsense script'; // here add your adsense code, between ''
    
    return $more_link . $extra_more;
}