threed's function works great, but the replacement for the letter small io (ё) needs to be changed from 
<?php
if ($c==184) { $t.=chr(209).chr(209); continue; };
?>

to
<?php
if ($c==184) { $t.=chr(209).chr(145); continue; };
?>

so, the final working result should look like this:
<?php
function win3utf($s) {
   for($i=0, $m=strlen($s); $i<$m; $i++)    {
       $c=ord($s[$i]);
       if ($c<=127) {$t.=chr($c); continue; }
       if ($c>=192 && $c<=207) {$t.=chr(208).chr($c-48); continue; }
       if ($c>=208 && $c<=239) {$t.=chr(208).chr($c-48); continue; }
       if ($c>=240 && $c<=255) {$t.=chr(209).chr($c-112); continue; }
       if ($c==184) { $t.=chr(209).chr(209); continue; };
       if ($c==168) { $t.=chr(208).chr(129); continue; };
   }
   return $t;
}
?>