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);
}