[WP] Shortcode for a custom Swiper gallery, extract & convert plain text image urls from WordPress post content

wp_gallery_shortcode_url_parser.php

<?php

// Setup Swiper before deploy
// http://idangero.us/swiper

// Add Slideshow Shortcode for WordPress
function slideshow_custom_shortcode($atts, $content = null ) {
    extract( shortcode_atts( array(
	    'plain_text_url' => 'no' // in case the urls are plain text urls, write yes in the parameter eg. [slideshow plain_text_url="yes"]plain text urls here[/slideshow]
    ), $atts ) );

    // If we only have a bunch of plain text urls (no a or img tags *sigh*)
    if($plain_text_url != 'no') {
        global $post;
        $doc = new DOMDocument;
        $doc->loadHTML(get_the_content($post->ID));
        $post_content = $doc->textContent;

        // Get only the urls in the slideshow shortcode
        $start = '[slideshow plain_text_url="yes"]';
        $end = '[/slideshow]';
        $post_content = ' ' . $post_content;
        $ini = strpos($post_content, $start);
        if ($ini == 0) return '';
        $ini += strlen($start);
        $len = strpos($post_content, $end, $ini) - $ini;
        $substring_full = substr($post_content, $ini, $len);

        // Get an array of urls
        $string_array = explode(PHP_EOL, $substring_full);
        $new_array = array();
        foreach ($string_array as $url) {
            if(strlen($url) > 1)
                /* This actually only works in some cases; other times, the Swiper gallery removes the bg image for some reason
                $new_array[] = '<div class="swiper-slide" style="background-image:url('.$url.')"></div>'; */
                $new_array[] = '<div class="swiper-slide"><img src="'.$url.'" /></div>';
        }

        // Take over the content
        $content = implode(PHP_EOL,$new_array);
    }

    // Output the code for the gallery
    return '<div class="swiper-container gallery-top">
                <div class="swiper-wrapper">
                    '.$content.'
                </div>
                <div class="swiper-button-next swiper-button-white"></div>
                <div class="swiper-button-prev swiper-button-white"></div>
            </div>
            <div class="swiper-container gallery-thumbs">
                <div class="swiper-wrapper">
                    '.$content.'
                </div>
            </div>';
}
add_shortcode( 'slideshow', 'slideshow_custom_shortcode' );

?>