Callback URL using $wp_rewrite->add_external_rule() not working

I am trying create a virtual url for my SSO callback plugin and using –

// SSO callbacks
    function web_sso_callback() {
        global $wp_rewrite;
        $plugin_url = plugins_url( 'authcallback.php', __FILE__ );
        $plugin_url = substr( $plugin_url, strlen( home_url() ) + 1 );

        // The pattern is prefixed with '^'
        // The substitution is prefixed with the "home root", at least a '/'
        // This is equivalent to appending it to <code>non_wp_rules</code>
        $wp_rewrite->add_external_rule( 'authcallback.php$', $plugin_url );
    }

    // init functions
    function web_init_action(){
        web_sso_callback();
    }
    add_action( 'init', 'web_init_action' );

However this is not working. authcallback.php sits in the root of the plugin folder. Any hints as to what i am doing wrong.