Как выделить строку, отличную от utf8, в тексте utf8 в PHP?

Наконец-то я принял рабочее решение. Отправьте ответ, если кто-то подойдет к тому же вопросу.

class Highlighter
{
    private $_text;
    private $_keywords;

    private $keywords;
    private $text;

    private $tag = "b";

    public function highlight($text, $keywords)
    {
        $this->text = $text;
        $this->keywords = (array) $keywords;

        if (count($keywords) > 0)
        {
            $this->prepareString();
            $this->highlightStrings();
        }

        return $this->text;
    }

    private function unicodeSymbols()
    {
        return [
            'ā' => 'a',
            'č' => 'c',
            'ē' => 'e',
            'ī' => 'i',
            'ķ' => 'k',
            'ļ' => 'l',
            'ņ' => 'n',
            'š' => 's',
            'ū' => 'u',
            'ž' => 'z'
        ];
    }

    private function clearVars()
    {
        $this->_text = null;
        $this->_keywords = [];
    }

    private function prepareString()
    {
        $this->clearVars();

        $this->_text = strtolower( strtr($this->text, $this->unicodeSymbols()) );

        foreach ($this->keywords as $keyword)
        {
            $this->_keywords[] = strtolower( strtr($keyword, $this->unicodeSymbols()) );
        }
    }

    private function highlightStrings()
    {
        foreach ($this->_keywords as $keyword)
        {

            if (strlen($keyword) === 0) continue;

            // find cleared keyword in cleared text.
            $pos = strpos($this->_text, $keyword);

            if ($pos !== false)
            {

                $keywordLength = strlen($keyword);

                // find original keyword.
                $originalKeyword = mb_substr($this->text, $pos, $keywordLength);

                // highlight in both texts.
                $this->text = str_replace($originalKeyword, "<{$this->tag}>".$originalKeyword."</{$this->tag}>", $this->text);
                $this->_text = str_replace($keyword, "<{$this->tag}>".$keyword."</{$this->tag}>", $this->_text);
            }

        }
    }

    public function setTag($tag)
    {
        $this->tag = $tag;
    }
}