To highlight words in multi-byte text:
<?php
$s = 'Алабала';
$f = 'а';
echo preg_replace('/('.$f.')/iu', '<b>$1</b>', $s);
?>
    (PHP 4 >= 4.2.0, PHP 5, PHP 7, PHP 8)
mb_eregi_replace — Replace regular expression with multibyte support ignoring case
$pattern
   , string $replacement
   , string $string
   , string|null $options = null
   ) : string|false|null
   Scans string for matches to
   pattern, then replaces the matched text
   with replacement. 
  
patternThe regular expression pattern. Multibyte characters may be used. The case will be ignored.
replacementThe replacement text.
stringThe searched string.
options
   The resultant string or false on error. 
   If string is not valid for the current encoding, null
   is returned.
  
| 版本 | 说明 | 
|---|---|
| 8.0.0 | optionsis nullable now. | 
| 7.1.0 | The function checks whether stringis valid for the
        current encoding. | 
| 7.1.0 | The emodifier has been deprecated. | 
Note:
mb_regex_encoding() 指定的内部编码或字符编码将会当作此函数用的字符编码。
处理非信任的输入时从不使用
e 修饰符,就不会转码(即调用 preg_replace())。不注意这些会很可能会导致应用程序引发远程代码执行的漏洞。
To highlight words in multi-byte text:
<?php
$s = 'Алабала';
$f = 'а';
echo preg_replace('/('.$f.')/iu', '<b>$1</b>', $s);
?>
    
when trying to find a way to strip newline from a multibyte UTF-8 string i got to this function just to discover later that POSIX don't "do" newline so i can't strip them, examples of what i tried are : \r\n , \\r\\n , (\\r\\n) (\\r|\\n) 
and got no result
so since i wanted something like mb_nl2br() that's simple i wrote this little recursive function for UTF-8:
<?php
function mb_str_replace($find,$replace,&$str)
{
$i = mb_strpos($str,$find, 0,"UTF-8");
if ($index===false) {return;}
$str = mb_substr($str, 0,$i).$replace.mb_substr($str, $i+mb_strlen($find,"UTF-8"),mb_strlen($str,"UTF-8"));
$this->mb_str_replace($find,$replace,$str);
}
?>
note: moderate unit tesing was done, changed to other encodings
    
Transliterator for cyrillic-to-latin letters for UTF chars:
<?php
function do_translit($st) {
    $replacement = array(
        "й"=>"i","ц"=>"c","у"=>"u","к"=>"k","е"=>"e","н"=>"n",
        "г"=>"g","ш"=>"sh","щ"=>"sh","з"=>"z","х"=>"x","ъ"=>"\'",
        "ф"=>"f","ы"=>"i","в"=>"v","а"=>"a","п"=>"p","р"=>"r",
        "о"=>"o","л"=>"l","д"=>"d","ж"=>"zh","э"=>"ie","ё"=>"e",
        "я"=>"ya","ч"=>"ch","с"=>"c","м"=>"m","и"=>"i","т"=>"t",
        "ь"=>"\'","б"=>"b","ю"=>"yu",
        "Й"=>"I","Ц"=>"C","У"=>"U","К"=>"K","Е"=>"E","Н"=>"N",
        "Г"=>"G","Ш"=>"SH","Щ"=>"SH","З"=>"Z","Х"=>"X","Ъ"=>"\'",
        "Ф"=>"F","Ы"=>"I","В"=>"V","А"=>"A","П"=>"P","Р"=>"R",
        "О"=>"O","Л"=>"L","Д"=>"D","Ж"=>"ZH","Э"=>"IE","Ё"=>"E",
        "Я"=>"YA","Ч"=>"CH","С"=>"C","М"=>"M","И"=>"I","Т"=>"T",
        "Ь"=>"\'","Б"=>"B","Ю"=>"YU",
    );
    
    foreach($replacement as $i=>$u) { 
        $st = mb_eregi_replace($i,$u,$st);
    }
    return $st;
}
?>