foreach ($countries as $country) {
  $country_id = $country['ID'];
  $country = get_the_title().", "; //comma

  echo "<pre>";var_dump($country);echo "</pre>";
  //string(6) "Taiwan, "
  //string(8) "Tanzania, "
  //string(14) "United Kingdom, "

  echo rtrim($country,", ");    //Works not: Taiwan, Tanzania, United Kingdom,
  echo substr($country, 0, strlen($country)-1); //Works not: Taiwan, Tanzania, United Kingdom,
  echo substr($country, 0, -1); //Works not: Taiwan, Tanzania, United Kingdom,
  echo implode(", ", $country); //Works not: Warning: implode(): Invalid arguments passed
  echo $country[-1] = PHP_EOL;  //Works not: (empty)
  echo substr($country, -1);    //Works not: (empty)
  echo substr(trim($country), 0, -1); //Works not: Taiwan Tanzania United Kingdom
  echo str_replace(', ', '', $country); //Works not: TaiwanTanzaniaUnited Kingdom
  echo str_replace('', ', ', $country); //Works not: Taiwan, Tanzania, United Kingdom,

}