This is edited functions utf8_to_cp1251 and cp1251_to_utf8. 
Changes: Check current string encoding.

<?php
function cp1251_to_utf8($s)
  {
  if ((mb_detect_encoding($s,'UTF-8,CP1251')) == "WINDOWS-1251")
    {
    $c209 = chr(209); $c208 = chr(208); $c129 = chr(129);
    for($i=0; $i<strlen($s); $i++)
      {
      $c=ord($s[$i]);
      if ($c>=192 and $c<=239) $t.=$c208.chr($c-48);
      elseif ($c>239) $t.=$c209.chr($c-112);
      elseif ($c==184) $t.=$c209.$c209;
      elseif ($c==168)    $t.=$c208.$c129;
      else $t.=$s[$i];
      }
    return $t;
    }
  else
    {
    return $s;
    }
   }

function utf8_to_cp1251($s)
  {
  if ((mb_detect_encoding($s,'UTF-8,CP1251')) == "UTF-8")
    {
    for ($c=0;$c<strlen($s);$c++)
      {
      $i=ord($s[$c]);
      if ($i<=127) $out.=$s[$c];
      if ($byte2)
        {
        $new_c2=($c1&3)*64+($i&63);
        $new_c1=($c1>>2)&5;
        $new_i=$new_c1*256+$new_c2;
        if ($new_i==1025)
          {
          $out_i=168;
          } else {
          if ($new_i==1105)
            {
            $out_i=184;
            } else {
            $out_i=$new_i-848;
            }
          }
        $out.=chr($out_i);
        $byte2=false;
        }
        if (($i>>5)==6)
          {
          $c1=$i;
          $byte2=true;
          }
      }
    return $out;
    }
  else
    {
    return $s;
    }
  }
?>

If you need convert string from Windows-1251 to 866. Some characters of 1251 haven't representation on DOS 866. For example, long dash -- chr(150) will be converted to 0, after that iconv finish his work and other charactes  will be skiped. Problem characters range in win1251 (128-159,163,165-167,169,171-174,177-182,187-190).
Use this:
//$text  -  input text in windows-1251
//$cout  -  output text in 866 (cp866, dos ru ascii)
for($i=0;$i<strlen($text);$i++) {
    $ord=ord($text[$i]);
    if($ord>=192&&$ord<=239) $cout.=chr($ord-64);
    elseif($ord>=240&&$ord<=255) $cout.=chr($ord-16);
    elseif($ord==168) $cout.=chr(240);
    elseif($ord==184) $cout.=chr(241);
    elseif($ord==185) $cout.=chr(252);
    elseif($ord==150||$ord==151) $cout.=chr(45);
    elseif($ord==147||$ord==148||$ord==171||$ord==187) $cout.=chr(34);
    elseif($ord>=128&&$ord<=190) $i=$i; //нет представления данному символу
    else $cout.=chr($ord);
}