Cut image on the fly by image ID, WordPress

functions.php

<?php 
function get_thumbnail($attachment_id = null, $width = null, $height = null, $crop = true, $url = false, $alt = "") {
  // global $post variable is used later in the function to check if the current post has a thumbnail image
  global $post;

  // if an attachment ID is provided
  if ($attachment_id) {
    // get the path to the file on the server
    $path = get_attached_file($attachment_id);
    // if the file does not exist, return false
    if (!file_exists($path)) {
      return false;
    }

    // get the base URL for the uploads directory and the directory of the file
    $upload = wp_upload_dir();
    $path_info = pathinfo($path);
    $base_url = $upload['baseurl'] . str_replace($upload['basedir'], '', $path_info['dirname']);

    // get the attachment metadata
    $meta = wp_get_attachment_metadata($attachment_id);
    // check if a size with the desired width and height already exists
    foreach ($meta['sizes'] as $size) {
      if ($size['width'] == $width && $size['height'] == $height) {
        // if a size with the desired dimensions exists, use it
        $img_src = "{$base_url}/{$size['file']}";
        break;
      }
    }

    // if a size with the desired dimensions does not already exist
    if (!isset($img_src)) {
      // try to create a new size with the desired dimensions
      $resized = image_make_intermediate_size($path, $width, $height, $crop);
      // if the resizing was successful and not an error
      if ($resized && !is_wp_error($resized)) {
        // add the new size to the attachment metadata
        $key = sprintf('resized-%dx%d', $width, $height);
        $meta['sizes'][$key] = $resized;
        wp_update_attachment_metadata($attachment_id, $meta);

        // use the new size
        $img_src = "{$base_url}/{$resized['file']}";
      } else {
        // if the resizing failed, use the original image
        $img_src = "{$base_url}/{$path_info['basename']}";
      }
    }
  } 
  // if no attachment ID is provided, but the global $post variable is set
  elseif (!empty($post)) {
    // if the post has a thumbnail image
    if (has_post_thumbnail($post->ID)) {
      // get the thumbnail ID and generate the thumbnail
      $attachment_id = get_post_thumbnail_id($post->ID);
      $img_src = get_thumbnail($attachment_id, $width, $height, $crop, true);
    } 
    // if no thumbnail image is set for the post
    else {
      // try to find an image in the post content
      $output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
      // if an image is found
      if ($output >= 1) {
        // use the first image found
        $img_src = $matches[1][0];
      } else {
        // if no image is found in the post, use a placeholder image
        $img_src = "https://via.placeholder.com/${width}x${height}";
      }
    }
  }
  // if no attachment ID is provided and no global $post variable is set
  else {
    // use a placeholder image
    $img_src = "https://via.placeholder.com/${width}x${height}";
  }
  
  // if the $url parameter is true, return the URL of the image
  if ($url) {
    return $img_src;
  } 
  // if the $url parameter is not true, return an <img> element with the image's URL as the src attribute
  else {
    return '<img src="' . $img_src . '" alt="' . $alt . '" height="' . $height . '" width="' . $width . '">';
  }
}