php str_replace无法替换单个字符

Question

我正在尝试将正常数字的字符串的单个字符替换为阿拉伯语。

我有

  $er_num = array('0','1','2','3','4','5','6','7','8','9');
  $ar_num = array('٠','١','٢','٣','٤','٥','٦','٧','٨','٩');
  echo str_replace($er_num,$ar_num,'12');

它适用于多位数..但是当它出现单个数字时，它没有...

  echo str_replace($er_num,$ar_num,'1');