Для конвертации на php строки из utf-8 в windows-1251 и наоборот, можно использовать следующую функцию:

$text = iconv('utf-8', 'windows-1251', $text);

если необходимо обратное действие, то:

$text = iconv('windows-1251', 'utf-8', $text);

Описание функции iconv:
string iconv ( string from_kodirovka, string to_kodirovka, string str )
Производит преобразование кодировки символов строки str из начальной кодировки from_kodirovka в конечную to_kodirovka. Возвращает строку в новой кодировке, или FALSE в случае ошибки.

Если добавить //TRANSLIT к параметру out_charset будет включена транслитеризация. Это означает, что вслучае, когда символа нет в конечной кодировке, он заменяется одним или несколькими аналогами. Если добавить //IGNORE, то символы, которых нет в конечной кодировке, будут опущены. Иначе, будет возвращена строка str, обрезанная до первого недопустимого символа.

В случае, если ваш хостинг не поддерживает iconv, для конвертации из utf-8 в win-1251 и наоборот можно использовать следующие функции:

function utf8_to_cp1251($s) {
    $tbl = $GLOBALS['unicode_to_cp1251_tbl'];
    $uc = 0;
    $bits = 0;
    $r = "";
    for($i = 0, $l = strlen($s); $i < $l; $i++) {
        $c = $s{$i};
        $b = ord($c);
        if($b &amp; 0x80) {
            if($b &amp; 0x40) {
                if($b &amp; 0x20) {
                    $uc = ($b &amp; 0x0F) << 12;
                    $bits = 12;
                } else {
                    $uc = ($b &amp; 0x1F) << 6;
                    $bits = 6;
                }
            } else {
                $bits -= 6;
                if($bits) {
                    $uc |= ($b &amp; 0x3F) << $bits;
                } else {
                    $uc |= $b &amp; 0x3F;
                    if($cc = @$tbl[$uc]) {
                        $r .= $cc;
                    } else {
                        $r .= '?';
                    }
                }
            }
        } else {
            $r .= $c;
        }
    }
    return $r;
}

function cp1251_to_utf8($s) {
    $tbl = $GLOBALS['cp1251_to_utf8_tbl'];
    $r = "";
    for($i = 0, $l = strlen($s); $i < $l; $i++) {
        $c = $s{$i};
        $b = ord($c);
        if ($b < 128) {
            $r .= $c;
        } else {
            $r .= @$tbl[$b];
        }
    }
    return $r; 
} 

$unicode_to_cp1251_tbl = array(
0x0402 => "\x80", 
0x0403 => "\x81", 
0x201A => "\x82", 
0x0453 => "\x83", 
0x201E => "\x84", 
0x2026 => "\x85", 
0x2020 => "\x86", 
0x2021 => "\x87", 
0x20AC => "\x88", 
0x2030 => "\x89", 
0x0409 => "\x8A", 
0x2039 => "\x8B", 
0x040A => "\x8C", 
0x040C => "\x8D", 
0x040B => "\x8E", 
0x040F => "\x8F", 
0x0452 => "\x90", 
0x2018 => "\x91", 
0x2019 => "\x92", 
0x201C => "\x93", 
0x201D => "\x94", 
0x2022 => "\x95", 
0x2013 => "\x96", 
0x2014 => "\x97", 
0x2122 => "\x99", 
0x0459 => "\x9A", 
0x203A => "\x9B", 
0x045A => "\x9C", 
0x045C => "\x9D", 
0x045B => "\x9E", 
0x045F => "\x9F", 
0x00A0 => "\xA0", 
0x040E => "\xA1", 
0x045E => "\xA2", 
0x0408 => "\xA3", 
0x00A4 => "\xA4", 
0x0490 => "\xA5", 
0x00A6 => "\xA6", 
0x00A7 => "\xA7", 
0x0401 => "\xA8", 
0x00A9 => "\xA9", 
0x0404 => "\xAA", 
0x00AB => "\xAB", 
0x00AC => "\xAC", 
0x00AD => "\xAD", 
0x00AE => "\xAE", 
0x0407 => "\xAF", 
0x00B0 => "\xB0", 
0x00B1 => "\xB1", 
0x0406 => "\xB2", 
0x0456 => "\xB3", 
0x0491 => "\xB4", 
0x00B5 => "\xB5", 
0x00B6 => "\xB6", 
0x00B7 => "\xB7", 
0x0451 => "\xB8", 
0x2116 => "\xB9", 
0x0454 => "\xBA", 
0x00BB => "\xBB", 
0x0458 => "\xBC", 
0x0405 => "\xBD", 
0x0455 => "\xBE", 
0x0457 => "\xBF", 
0x0410 => "\xC0", 
0x0411 => "\xC1", 
0x0412 => "\xC2", 
0x0413 => "\xC3", 
0x0414 => "\xC4", 
0x0415 => "\xC5", 
0x0416 => "\xC6", 
0x0417 => "\xC7", 
0x0418 => "\xC8", 
0x0419 => "\xC9", 
0x041A => "\xCA", 
0x041B => "\xCB", 
0x041C => "\xCC", 
0x041D => "\xCD", 
0x041E => "\xCE", 
0x041F => "\xCF", 
0x0420 => "\xD0", 
0x0421 => "\xD1", 
0x0422 => "\xD2", 
0x0423 => "\xD3", 
0x0424 => "\xD4", 
0x0425 => "\xD5", 
0x0426 => "\xD6", 
0x0427 => "\xD7", 
0x0428 => "\xD8", 
0x0429 => "\xD9", 
0x042A => "\xDA", 
0x042B => "\xDB", 
0x042C => "\xDC", 
0x042D => "\xDD", 
0x042E => "\xDE", 
0x042F => "\xDF", 
0x0430 => "\xE0", 
0x0431 => "\xE1", 
0x0432 => "\xE2", 
0x0433 => "\xE3", 
0x0434 => "\xE4", 
0x0435 => "\xE5", 
0x0436 => "\xE6", 
0x0437 => "\xE7", 
0x0438 => "\xE8", 
0x0439 => "\xE9", 
0x043A => "\xEA", 
0x043B => "\xEB", 
0x043C => "\xEC", 
0x043D => "\xED", 
0x043E => "\xEE", 
0x043F => "\xEF", 
0x0440 => "\xF0", 
0x0441 => "\xF1", 
0x0442 => "\xF2", 
0x0443 => "\xF3", 
0x0444 => "\xF4", 
0x0445 => "\xF5", 
0x0446 => "\xF6", 
0x0447 => "\xF7", 
0x0448 => "\xF8", 
0x0449 => "\xF9", 
0x044A => "\xFA", 
0x044B => "\xFB", 
0x044C => "\xFC", 
0x044D => "\xFD", 
0x044E => "\xFE", 
0x044F => "\xFF",
); 

$cp1251_to_utf8_tbl = array( 
0x80 => "\xD0\x82", 
0x81 => "\xD0\x83", 
0x82 => "\xE2\x80\x9A", 
0x83 => "\xD1\x93", 
0x84 => "\xE2\x80\x9E", 
0x85 => "\xE2\x80\xA6", 
0x86 => "\xE2\x80\xA0", 
0x87 => "\xE2\x80\xA1", 
0x88 => "\xE2\x82\xAC", 
0x89 => "\xE2\x80\xB0", 
0x8A => "\xD0\x89", 
0x8B => "\xE2\x80\xB9", 
0x8C => "\xD0\x8A", 
0x8D => "\xD0\x8C", 
0x8E => "\xD0\x8B", 
0x8F => "\xD0\x8F", 
0x90 => "\xD1\x92", 
0x91 => "\xE2\x80\x98", 
0x92 => "\xE2\x80\x99", 
0x93 => "\xE2\x80\x9C", 
0x94 => "\xE2\x80\x9D", 
0x95 => "\xE2\x80\xA2", 
0x96 => "\xE2\x80\x93", 
0x97 => "\xE2\x80\x94", 
0x99 => "\xE2\x84\xA2", 
0x9A => "\xD1\x99", 
0x9B => "\xE2\x80\xBA", 
0x9C => "\xD1\x9A", 
0x9D => "\xD1\x9C", 
0x9E => "\xD1\x9B", 
0x9F => "\xD1\x9F", 
0xA0 => "\xC2\xA0", 
0xA1 => "\xD0\x8E", 
0xA2 => "\xD1\x9E", 
0xA3 => "\xD0\x88", 
0xA4 => "\xC2\xA4", 
0xA5 => "\xD2\x90", 
0xA6 => "\xC2\xA6", 
0xA7 => "\xC2\xA7", 
0xA8 => "\xD0\x81", 
0xA9 => "\xC2\xA9", 
0xAA => "\xD0\x84", 
0xAB => "\xC2\xAB", 
0xAC => "\xC2\xAC", 
0xAD => "\xC2\xAD", 
0xAE => "\xC2\xAE", 
0xAF => "\xD0\x87", 
0xB0 => "\xC2\xB0", 
0xB1 => "\xC2\xB1", 
0xB2 => "\xD0\x86", 
0xB3 => "\xD1\x96", 
0xB4 => "\xD2\x91", 
0xB5 => "\xC2\xB5", 
0xB6 => "\xC2\xB6", 
0xB7 => "\xC2\xB7", 
0xB8 => "\xD1\x91", 
0xB9 => "\xE2\x84\x96", 
0xBA => "\xD1\x94", 
0xBB => "\xC2\xBB", 
0xBC => "\xD1\x98", 
0xBD => "\xD0\x85", 
0xBE => "\xD1\x95", 
0xBF => "\xD1\x97", 
0xC0 => "\xD0\x90", 
0xC1 => "\xD0\x91", 
0xC2 => "\xD0\x92", 
0xC3 => "\xD0\x93", 
0xC4 => "\xD0\x94", 
0xC5 => "\xD0\x95", 
0xC6 => "\xD0\x96", 
0xC7 => "\xD0\x97", 
0xC8 => "\xD0\x98", 
0xC9 => "\xD0\x99", 
0xCA => "\xD0\x9A", 
0xCB => "\xD0\x9B", 
0xCC => "\xD0\x9C", 
0xCD => "\xD0\x9D", 
0xCE => "\xD0\x9E", 
0xCF => "\xD0\x9F", 
0xD0 => "\xD0\xA0", 
0xD1 => "\xD0\xA1", 
0xD2 => "\xD0\xA2", 
0xD3 => "\xD0\xA3", 
0xD4 => "\xD0\xA4", 
0xD5 => "\xD0\xA5", 
0xD6 => "\xD0\xA6", 
0xD7 => "\xD0\xA7", 
0xD8 => "\xD0\xA8", 
0xD9 => "\xD0\xA9", 
0xDA => "\xD0\xAA", 
0xDB => "\xD0\xAB", 
0xDC => "\xD0\xAC", 
0xDD => "\xD0\xAD", 
0xDE => "\xD0\xAE", 
0xDF => "\xD0\xAF", 
0xE0 => "\xD0\xB0", 
0xE1 => "\xD0\xB1", 
0xE2 => "\xD0\xB2", 
0xE3 => "\xD0\xB3", 
0xE4 => "\xD0\xB4", 
0xE5 => "\xD0\xB5", 
0xE6 => "\xD0\xB6", 
0xE7 => "\xD0\xB7", 
0xE8 => "\xD0\xB8", 
0xE9 => "\xD0\xB9", 
0xEA => "\xD0\xBA", 
0xEB => "\xD0\xBB", 
0xEC => "\xD0\xBC", 
0xED => "\xD0\xBD", 
0xEE => "\xD0\xBE", 
0xEF => "\xD0\xBF", 
0xF0 => "\xD1\x80", 
0xF1 => "\xD1\x81", 
0xF2 => "\xD1\x82", 
0xF3 => "\xD1\x83", 
0xF4 => "\xD1\x84", 
0xF5 => "\xD1\x85", 
0xF6 => "\xD1\x86", 
0xF7 => "\xD1\x87", 
0xF8 => "\xD1\x88", 
0xF9 => "\xD1\x89", 
0xFA => "\xD1\x8A", 
0xFB => "\xD1\x8B", 
0xFC => "\xD1\x8C", 
0xFD => "\xD1\x8D", 
0xFE => "\xD1\x8E", 
0xFF => "\xD1\x8F", 
);