Whole Zip file with all relevant functions

zip.php

<?php
/*
Plugin Name: Custom Functions for Screenpartners Bildebank
*/

/******************************************************
* TASK: Create LOW RES duplicates of uploaded images, *
* create alternate download button for these files    *
* "Download Files" / "Download Low Res Files"         *
******************************************************/

/* Directory Location of WP Download Manager's Uploads
** All wpdm files uploads to this directory */
function get_full_upload_dir() {
	$uploads = wp_upload_dir();
	return $uploads['baseurl'] . '/download-manager-files/';
}

function get_lres_full_upload_dir() {
	$uploads = wp_upload_dir();
	return $uploads['baseurl'] . '/lres-download-manager-files/';
}

// Function that returns an array of file paths to the relevant wpdm package
function uploaded_files_path() {
	$identity = get_the_ID();
    $files = get_package_data($identity, 'files');
    $file_list = array();

    // Adds each file path to $file_list array
    foreach ($files as $file) {
    	$file_path = UPLOAD_DIR . $file;
        $file_list[] = $file_path;
    }
    return $file_list;
}

// Function to create low res versions of images - returns an array with their path
function resize_images() {
	$file_list = uploaded_files_path();
	$low_res_image_path_list=array();

	foreach ($file_list as $file) {
		$imagick = new Imagick(); 
		$img = wp_get_image_editor($file);
		$imagick->readImage($file);
		
		if ( ! is_wp_error( $imagick ) ) {
			$filenamelres = $img->generate_filename('low-res', ABSPATH.'wp-content/uploads/lres-download-manager-files/', NULL );
			if (!file_exists($filenamelres)) {
				$imagick->setImageResolution(50,50); 
				$imagick->resampleImage(50,50,imagick::FILTER_UNDEFINED,0);
				$imagick->resizeImage( 600, 0, Imagick::FILTER_UNDEFINED, 1 );
				$imagick->writeImage($filenamelres);
				$low_res_image_path_list[] = $filenamelres;
			} else {
				$filenamelres = $img->generate_filename('low-res', ABSPATH.'wp-content/uploads/lres-download-manager-files/', NULL );
				$low_res_image_path_list[] = $filenamelres;
			}
		} else {
			// For testing
			echo 'ERROR! File ' . $count . ' might not be recognized as an image!';
			echo '<br>';
		}

		$imagick->destroy(); 
	}
	return $low_res_image_path_list;
}

function Zip($source, $destination) {

	echo "called function";

    if (!extension_loaded('zip') || !file_exists($source)) {
        return false;
    }

    $zip = new ZipArchive();
    if (!$zip->open($destination, ZIPARCHIVE::CREATE)) {
        return false;
    }

    $source = str_replace('\\', '/', realpath($source));

    if (is_dir($source) === true)
    {
        $files = new RecursiveIteratorIterator(new RecursiveDirectoryIterator($source), RecursiveIteratorIterator::SELF_FIRST);

        foreach ($files as $file)
        {
            $file = str_replace('\\', '/', $file);

            // Ignore "." and ".." folders
            if( in_array(substr($file, strrpos($file, '/')+1), array('.', '..')) )
                continue;

            $file = realpath($file);

            if (is_dir($file) === true)
            {
                $zip->addEmptyDir(str_replace($source . '/', '', $file . '/'));
            }
            else if (is_file($file) === true)
            {
                $zip->addFromString(str_replace($source . '/', '', $file), file_get_contents($file));
            }
        }
    }
    else if (is_file($source) === true)
    {
        $zip->addFromString(basename($source), file_get_contents($source));
    }

    return $zip->close();
}

// Function to create a download link for the created zip archive
function lres_download_link() {

	// Testing
	$resize_images = resize_images();
    $lres_directory = get_lres_full_upload_dir() . get_the_title() . '-low-res.zip';

	$filename = create_zip_lres($resize_images, $lres_directory);

	echo '<a href="' . $filename . '" alt="Download low res zip">Download Low res</a>';
}

<?php
        $resize_images = resize_images();
        $lres_package_title = preg_replace('/\s/', '', get_the_title());
        $lres_directory = ABSPATH.'wp-content/uploads/lres-download-manager-files/' . $lres_package_title . '-low-res.zip';


        resize_images(); 

        print_r($resize_images[0]); ?>


        <?php if (function_exists('Zip')) {
            Zip($resize_images[0], $lres_directory, true);
        } else {

        }
?>