Get post embedded image caption

Question

I am trying to get the caption of post embedded images to then use on a lightbox.

This is my code:

function my_theme_prefix_content_gallery( $content ) {
    global $post;

    $args = array(
        'post_type' => 'attachment',
        'post_mime_type'=>'image',
        'numberposts' => 1,
        'post_status' => null,
        'post_parent' => $post->ID,
    );

    $attachments = get_posts( $args );
    if ( $attachments ) {
        foreach ( $attachments as $attachment ) {
           $caption = $attachment->post_excerpt;
        }
    }

    $pattern     = "/<a(.*?)href=('|\")(.*?).(bmp|gif|jpeg|jpg|png)('|\")(.*?)>/i";
    $replacement = '<a$1href=$2$3.$4$5 rel="lightbox" title="' . $caption . '"$6>';
    $content     = preg_replace( $pattern, $replacement, $content );

    return $content;
}
add_filter( 'the_content', 'my_theme_prefix_content_gallery' );

What i have at the moment(code above) is giving me the caption from other images on the media library. How do I get the caption of the image that I am seeing?

EDIT

The Magnific Popup bit for the embedded images popup:

 $('a[rel="lightbox"]').magnificPopup ({
        type: 'image',
        mainClass: 'mfp-fade',
        fixedContentPos: false,
        removalDelay: 350,
        gallery: {
            enabled: true
        }
});

So whit this code I am being able to open post embedded images in Magnific Popup when clicked. All I am missing is a way to show their captions when they open in Magnific Popup.

Answer 1

If I understood correctly, you are trying to add, to every image embedded in a post gallery, a custom rel attribute and a title attribute based on the attachment caption. Instead of parsing the content, you can try to modify the anchor that wraps the images using the 'wp_get_attachment_link' filter.

<?php

    function wpse221533_add_caption_title_to_content_gallery_image_attachments( $markup, $id, $size, $permalink, $icon, $text ) {

        //Target only images
        if ( ! wp_attachment_is_image( $id ) ) :
            return $markup;
        endif;

        //Limit the scope of the new attributes
        $post_types = array( 'post', 'page' );

        if ( ! is_singular( $post_types ) ) :
            return $markup;
        endif;

        //Get attachment data
        $current_attachment_object = get_post( $id );

        //Get attachment caption
        $current_attachment_caption = $current_attachment_object->post_excerpt;

        //Nothing wrong with regex, but str_replace is cheaper
        $markup = str_replace( ' href=', ' rel="lightbox" title="' . $current_attachment_caption . '" href=', $markup );

        return $markup;

    }

    add_filter( 'wp_get_attachment_link', 'wpse221533_add_caption_title_to_content_gallery_image_attachments', 10, 6 ) ;

?>

EDIT 16/04/06

In fact, I didn't understand correctly, since I was referring to posts' image galleries and the op to posts' single image attachments. The code above will only work for galleries.

This other code will do it for single image attachments:

<?php

    function wpse221533_add_caption_title_to_content_single_image_attachments( $content ) {

        //Limit the scope of the new attributes
        $post_types = array( 'post', 'page' );

        if ( ! is_singular( $post_types ) ) :
            return $content;
        endif;

        //Parse the content DOM for links
        $doc = new DOMDocument();
        $doc->loadHTML( $content );

        $anchors = $doc->getElementsByTagName( 'a' ); 

        //Parse each link
        foreach ( $anchors as $anchor ) :

            //Get the rel attribute
            $anchor_rel = $anchor->getAttribute( 'rel' );

            //Get the image ID based on the rel attribute
            $current_attachment_id = substr( strrchr( $anchor_rel, "-" ), 1 );

            //Check if the extracted ID is actually a number
            if ( ! is_numeric( $current_attachment_id ) ) :
                return $content;
            endif;

            //Target only images
            if ( ! wp_attachment_is_image( $current_attachment_id ) ) :
                return $content;
            endif;

            //Get the attachment object
            $current_attachment_object = get_post( $current_attachment_id );

            //Get the attachment caption
            $current_attachment_caption = $current_attachment_object->post_excerpt;

            //Set the rel attribute
            $anchor->setAttribute( 'rel', 'lightbox' );

            //Finally set the title attribute based on the attachment caption
            $anchor->setAttribute( 'title', $current_attachment_caption );

        endforeach;

        //Save the DOM changes
        $content = $doc->saveHTML();

        return $content;

    }

    add_filter( 'the_content', 'wpse221533_add_caption_title_to_content_single_image_attachments', 10, 1 );

?>