Pasted

<?php
    
    // post ID
    $post_id = 123;
    // reference of the post, the images will be uploaded in a folder named after it, could be replaced by the post ID if you don't have a reference
    $reference = 'TEST-REF';
    // array of images URLs to attach
    $images = array(
        'https://static5.immomigsa.ch/s/710e6/1/pictures/objects/1_5e6895ada6a599.35609643.jpg',
        'https://static3.immomigsa.ch/s/c9e4c/1/pictures/objects/1_5e6895ade6a061.09425577.jpg',
        'https://static3.immomigsa.ch/s/11761/1/pictures/objects/1_5e6895ae1bba80.89092542.jpg',
    );
    
    // key of the ACF to update
    $acf_key = 'images';
    
    // declaring upload base directory, images will be uploaded within subfolder of this folder, created un the "uploads" folder
    $base_upload_folder = 'immomig';
    
    // getting WP uploads path
    $wp_upload_path = wp_upload_dir()['basedir'];
    // if base folder doesn't exist, it is created
    if (!is_dir($wp_upload_path.'/'.$base_upload_folder.'/'))
        mkdir($wp_upload_path.'/'.$base_upload_folder.'/');
    // removing the previously attached images
    $attachedz = get_field_object($acf_key, $post_id, false, true); // getting the list of images linked to this post through this ACF
    if ($attachedz[0]) { // if there are several in the main array...
        foreach($attachedz as $attached) // ...then for each...
            wp_delete_attachment(is_numeric($attached['value'])?$attached['value']:$attached['ID'], true); // ...we delete it
    }
    elseif (is_array($attachedz['value'])) { // or if there are several in a sub array...
        foreach($attachedz['value'] as $attached_id) // ...then for each...
            wp_delete_attachment($attached_id, true); // ...we delete it
    }
    else { // or if there is a single one...
        wp_delete_attachment(is_numeric($attachedz['value'])?$attachedz['value']:$attachedz['ID'], true); // ...we delete it
    }
    // initiating array of attached images IDs
    $attach_idz = array();
    // looping on each image URL
    foreach($images as $url) {
        // getting filename
        $filename = basename($url);
        // getting upload path directory
        $upload_path_dir = $wp_upload_path.'/'.$base_upload_folder.'/'.$reference.'/';
        // if directory doesn't existe, it is created
        if (!is_dir($upload_path_dir))
            mkdir($upload_path_dir);
        // getting complete upload path for the file
        $upload_path = $upload_path_dir.$filename;
        // if a file with the same name already exists, it is renamed with a trailing number
        $n = 2;
        while(is_file($upload_path))
            $upload_path = $upload_path_dir.pathinfo($filename,PATHINFO_FILENAME).'-'.($n++).'.'.pathinfo($filename,PATHINFO_EXTENSION);
        // uploading file to folder
        file_put_contents($upload_path, file_get_contents($url));
        // getting file type
        $filetype = wp_check_filetype(basename($upload_path), null);
        // iniatiating attachment datas
        $attachment = array(
            'guid'           => $upload_path, 
            'post_mime_type' => $filetype['type'],
            'post_title'     => preg_replace('/\.[^.]+$/', '', $filename),
            'post_content'   => '',
            'post_status'    => 'inherit'
        );
        // attaching the file to the post
        $attach_id = wp_insert_attachment($attachment, $upload_path, $post_id);
        // loading admin image library
        require_once(ABSPATH.'wp-admin/includes/image.php');
        // genertating attachment metadata
        $attach_data = wp_generate_attachment_metadata($attach_id, $upload_path);
        // updating metadata on the attached image
        wp_update_attachment_metadata($attach_id, $attach_data);
        // adding attachment ID to the array of attached images IDs
        $attach_idz[] = $attach_id;
    }
    
    // updating ACF gallery with images IDs
    update_field($acf_key, $attach_idz, $post_id);