responsive thumbnails for wordpress

responsive_post_thumbnail.php

/*
 * crohrer_responsive_post_thumbnail
 *
 * @author: Christoph Rohrer
 *
 */
function crohrer_responsive_post_thumbnail( $post_id, $sizesAttribute = '100vw', $thumbnailSizes = array(), $alt = '', $class = '' )
{
    $postThumbnailId = get_post_thumbnail_id( $post_id );
    if(empty($thumbnailSizes)){
        $thumbnailSizes = get_intermediate_image_sizes(); // fallback to all available sizes
    }
    $srcsets = array();
    $thumbnails = array();
    foreach ($thumbnailSizes as $thumbnailSize){
        $thumbnail = wp_get_attachment_image_src($postThumbnailId, $thumbnailSize );
        // only use properly resized images
        if($thumbnail[3]){
            $thumbnails[$thumbnailSize] = $thumbnail[1];
            $srcsets[$thumbnail[1]] = $thumbnail[0].' '.$thumbnail[1].'w';
        }
    }
    ksort($srcsets);
    $smallestSize = array_keys($thumbnails, min($thumbnails))[0];
    $image  = '<img src="'.wp_get_attachment_image_src($postThumbnailId, $smallestSize)[0].'"';
    $image .= ' alt="'.$alt.'"';
    $image .= (!empty($class)) ? ' class="'.$class.'"' : '';
    $image .= ' srcset="'.implode(', ', $srcsets).'"';
    $image .= ' sizes="'.$sizesAttribute.'"/>';

    return $image;
}