You can achieve that by appending "//IGNORE" after the codeset name to which the string is going to be converted.

For example:
<?php
  $bar = iconv("UTF-8", "KOI-8R//IGNORE", $foo);
?>

Note that this is not portable since most of the iconv implementations don't support it. As far as I know, only glibc's iconv can handle this.

it is said libxml2 does it this way(into numeric entities)
using iconv, means that it's possible, but i'm not sure

if it's possible, i guess it should be ok for php itself to implement "//IGNORE"
simply scan for //IGNORE itself, then do copies whenever get unconvertable error

it's badly needed to avoid truncate to the content only 1 char is unconvertable.
many thanks

Below is PHP example of how such code may looks like. It converts given string from UTF-8 into specified encoding. 
Notice about difference between utf8ToEntities() and utf8ToEntitiesMultibyte(): first function converts every char 
in a string into numeric entity while second only converts chars with codes above 0x0800. It allows for example 
receive normal string with single numeric entity in a case, when there is only one uncovertable character in it.

// Convert string from UTF-8 into specified encoding and substitute unconvertable characters by numeric entities
// At enter:
//   $str - string to convert
    function fromUTF8($str,$encoding)
    {
        if ($str===null)
            return(null);
        $t = iconv('utf-8',$encoding,$str);
        if (($t=='') && ($str!=''))
// iconv() is unable to convert this string into requested encoding.
        {
// First of all try to convert only multibyte characters. It may help us to return text in requested encoding
// with only exception of a few very special chars instead of having all text to be converted in entities.
            $str2 = utf8ToEntitiesMultibyte($str);
            $t = iconv('utf-8',$encoding,$str2);
            if ($t!='')
                return($t);
            else
                return(utf8ToEntities($str));
        };
        return($t);
    }

// Convert multibyte characters, available into UTF-8 encoded string into numeric entities (as described into RFC 2044)
// At enter:
//   $str - string into UTF-8 encoding
    function utf8ToEntitiesMultibyte($str)
    {
        if (!is_string($str))
            return('');
        $i = 0;
        $output = '';
        while($i<strlen($str))
        {
            $char = $str{$i};
            if ((ord($char) & 0x80)==0)
//   0000 0000-0000 007F   0xxxxxxx
                {
                    $output .= $char;
                     $i++;
                }
            elseif ((ord($char)>0xC0) && (ord($char)<=0xDF))
//   0000 0080-0000 07FF   110xxxxx 10xxxxxx
                {
                    $output .= substr($str,$i,2);
                    $i += 2;
                }
            else
                {
                    $num = 0;
                    if ((ord($char) & 0xFC)==0xFC)
//   0400 0000-7FFF FFFF   1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
                        {
                            $num = (ord($str{$i+5}) & 0x3F) |
                                  ((ord($str{$i+4}) & 0x3F) << 6 ) |
                                  ((ord($str{$i+3}) & 0x3F) << 12) |
                                  ((ord($str{$i+2}) & 0x3F) << 18) |
                                  ((ord($str{$i+1}) & 0x3F) << 24) |
                                  ((ord($str{$i+0}) & 0x01) << 30);
                            $i += 6;
                        }
                    elseif ((ord($char) & 0xF8)==0xF8)
//   0020 0000-03FF FFFF   111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
                        {
                            $num = (ord($str{$i+4}) & 0x3F) |
                                  ((ord($str{$i+3}) & 0x3F) << 6 ) |
                                  ((ord($str{$i+2}) & 0x3F) << 12) |
                                  ((ord($str{$i+1}) & 0x3F) << 18) |
                                  ((ord($str{$i+0}) & 0x03) << 24);
                            $i += 5;
                        }
                    elseif ((ord($char) & 0xF0)==0xF0)
//   0001 0000-001F FFFF   11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
                        {
                            $num = (ord($str{$i+3}) & 0x3F) |
                                  ((ord($str{$i+2}) & 0x3F) << 6 ) |
                                  ((ord($str{$i+1}) & 0x3F) << 12) |
                                  ((ord($str{$i+0}) & 0x07) << 18);
                            $i += 4;
                        }
                    elseif ((ord($char) & 0xE0)==0xE0)
//   0000 0800-0000 FFFF   1110xxxx 10xxxxxx 10xxxxxx
                        {
                            $num = (ord($str{$i+2}) & 0x3F) |
                                  ((ord($str{$i+1}) & 0x3F) << 6 ) |
                                  ((ord($str{$i+0}) & 0x0F) << 12);
                            $i += 3;
                        }
                    else
// We should never came here until passed string is not UTF-8,
// but without this we're risk to fall in endless loop
                        {
                            $num = ord($char);
                            $i++;
                        };
                    $output .= '&#'.$num.';';
                };
        };
        return($output);
    }

// Convert UTF-8 encoded string into numeric entities (as described into RFC 2044)
// At enter:
//   $str - string into UTF-8 encoding
    function utf8ToEntities($str)
    {
        if (!is_string($str))
            return('');
        $i = 0;
        $output = '';
        while($i<strlen($str))
        {
            $char = $str{$i};
            if ((ord($char) & 0x80)==0)
//   0000 0000-0000 007F   0xxxxxxx
                {
                    $output .= $char;
                     $i++;
                }
            else
                {
                    $num = 0;
                    if ((ord($char) & 0xFC)==0xFC)
//   0400 0000-7FFF FFFF   1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
                        {
                            $num = (ord($str{$i+5}) & 0x3F) |
                                  ((ord($str{$i+4}) & 0x3F) << 6 ) |
                                  ((ord($str{$i+3}) & 0x3F) << 12) |
                                  ((ord($str{$i+2}) & 0x3F) << 18) |
                                  ((ord($str{$i+1}) & 0x3F) << 24) |
                                  ((ord($str{$i+0}) & 0x01) << 30);
                            $i += 6;
                        }
                    elseif ((ord($char) & 0xF8)==0xF8)
//   0020 0000-03FF FFFF   111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
                        {
                            $num = (ord($str{$i+4}) & 0x3F) |
                                  ((ord($str{$i+3}) & 0x3F) << 6 ) |
                                  ((ord($str{$i+2}) & 0x3F) << 12) |
                                  ((ord($str{$i+1}) & 0x3F) << 18) |
                                  ((ord($str{$i+0}) & 0x03) << 24);
                            $i += 5;
                        }
                    elseif ((ord($char) & 0xF0)==0xF0)
//   0001 0000-001F FFFF   11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
                        {
                            $num = (ord($str{$i+3}) & 0x3F) |
                                  ((ord($str{$i+2}) & 0x3F) << 6 ) |
                                  ((ord($str{$i+1}) & 0x3F) << 12) |
                                  ((ord($str{$i+0}) & 0x07) << 18);
                            $i += 4;
                        }
                    elseif ((ord($char) & 0xE0)==0xE0)
//   0000 0800-0000 FFFF   1110xxxx 10xxxxxx 10xxxxxx
                        {
                            $num = (ord($str{$i+2}) & 0x3F) |
                                  ((ord($str{$i+1}) & 0x3F) << 6 ) |
                                  ((ord($str{$i+0}) & 0x0F) << 12);
                            $i += 3;
                        }
                    elseif ((ord($char) & 0xC0)==0xC0)
//   0000 0080-0000 07FF   110xxxxx 10xxxxxx
                        {
                            $num = (ord($str{$i+1}) & 0x3F) |
                                  ((ord($str{$i+0}) & 0x1F) << 6 );
                            $i += 2;
                        }
                    else
// We should never came here until passed string is not UTF-8,
// but without this we're risk to fall in endless loop
                        {
                            $num = ord($char);
                            $i++;
                        };
                    $output .= '&#'.$num.';';
                };
        };
        return($output);
    }