Adding custom input for data attribute in insert media modal

I have taken a look at the source, unfortunately I haven’t seen a nice way to pass the information without saving it. Which sucks – big time – because this really isn’t anything that needs to be saved.

A workaround would be to enable PHP Sessions by putting the following at the beginning of your functions.php:

    if (!session_id()) {
        session_start();
    }

Now you are able to use $_SESSION variables, like this:

    $_SESSION[ 'your-key' ] = 'your-value';

Create your form field like this:

    function wpse_154330_attachment_fields_to_edit( $form_fields, $post ) {
        $current_screen = get_current_screen();
        // we are not saving, so no need to show the field on the attachment page
        if ( $current_screen->id == 'attachment' ) {
            return $form_fields;
        }
        $form_fields['fancyboxGroup'] = array(
            'label' => 'fancybox group',
            'input' => 'text',
            'value' => '', // leave the value empty
            'helps' => 'use this to group images in fancybox',
        );
        return $form_fields;
    }
    add_filter(
        'attachment_fields_to_edit',
        'wpse_154330_attachment_fields_to_edit',
        10,
        2
    );

Make use of the session variable like this:

    function wpse154330_save_attachment_field( $post, $attachment ) {
        // we're only setting up the variable, not changing anything else
        if ( isset( $attachment[ 'fancyboxGroup' ] ) {
            $_SESSION[ 'fancyboxGroup' ] = $attachment[ 'fancyboxGroup' ];
        }
        return $post;
    }
    add_filter(
        'attachment_fields_to_save',
        'wpse154330_save_attachment_field',
        10,
        2
    );

Modify the output accordingly:

    function wpse154330_image_send_to_editor(
        $html,
        $id,
        $caption,
        $title,
        $align,
        $url,
        $size,
        $alt=""
    ) {
        // no need to modify the output, if no fancybox group is given
        if (
            empty( $_SESSION[ 'fancyboxGroup' ] )
            || ! isset( $_SESSION[ 'fancyboxGroup' ] )
        ) {
            return $html;
        }
        $classes="fancybox";
        if ( preg_match( '/<a.*? class=".*?">/', $html ) ) {
            $html = preg_replace(
                '/(<a.*? class=".*?)(".*?>)/',
                '$1 ' . $classes . '$2',
                $html
            );
        } else {
            $html = preg_replace(
                '/(<a.*?)>/',
                '$1 class="'
                    . $classes
                    . '" data-fancybox-group="'
                    . $_SESSION[ 'fancyboxGroup' ]
                    . '" >',
                $html
            );
        }
        unset( $_SESSION[ 'fancyboxGroup' ] );
        return $html;
    }
    add_filter(
        'image_send_to_editor',
        'wpse154330_image_send_to_editor',
        10,
        8
    );

Thats about it. Should be pretty much self-explaining, otherwise just ask.