wp custom keys

get_post_custom_keys.php

<?php
  $sources = array();
  $keys = get_post_custom_keys();

  foreach ((array)$keys as $key) {
    $keyt = trim($key);
    
    if (is_protected_meta($keyt, 'post'))
      continue;
    $values = array_map('trim', get_post_custom_values($key));
    $value = implode($values, ', ');
    
    if ($key == "img_src_lg") {
      $sources[1] = '<source media="(min-width: 1200px)" srcset="' . $value . '">';
    } elseif ($key == "img_src_md") {
      $sources[2] = '<source media="(min-width: 992px)" srcset="' . $value . '">';
    } elseif ($key == "img_src_sm") {
      $sources[3] = '<source media="(min-width: 768px)" srcset="' . $value . '">';
    } elseif ($key == "img_src_xs") {
      $sources[4] = '<img srcset="' . $value . '" alt="">';
    }
  }

  ksort($sources);

  foreach ($sources as $order => $src) {
    echo $src;
  }