Class that handles string operations lowercase, uppercase, ucfirst, and ucwords in Turkish, solves the Turkish İ problem.

TurkishString.php

<?php

/*
* Package: https://github.com/epigra/trstringhelper
*/
class TurkishString
{
    /**
     * @var array
     */
    private static $alphabet = [
        'B'  => ['I', 'Ğ', 'Ü', 'Ş', 'İ', 'Ö', 'Ç'],
        'k'  => ['ı', 'ğ', 'ü', 'ş', 'i', 'ö', 'ç'],
        'Bi' => ['I', 'i'],
        'ki' => ['ı', 'İ']
    ];

    /**
     * @param $string
     *
     * @return string
     */
    public static function toLower($string) : string
    {
        return mb_strtolower(str_replace(static::$alphabet['B'], static::$alphabet['k'], $string), 'utf-8');
    }

    /**
     * @param $string
     *
     * @return string
     */
    public static function toUpper($string) : string
    {
        return mb_strtoupper(str_replace(static::$alphabet['k'], static::$alphabet['B'], $string), 'utf-8');
    }

    /**
     * @param $string
     *
     * @return string
     */
    public static function ucFirst($string) : string
    {
        return mb_strtoupper(mb_substr(str_replace(static::$alphabet['k'], static::$alphabet['B'], $string), 0, 1, 'utf-8'), 'utf-8') . mb_strtolower(mb_substr(str_replace(static::$alphabet['B'], static::$alphabet['k'], $string), 1, mb_strlen($string, 'utf-8') - 1, 'utf-8'));
    }

    /**
     * @param $string
     *
     * @return string
     */
    public static function ucWords($string) : string
    {
        return mb_convert_case(ltrim(str_replace(static::$alphabet['Bi'], static::$alphabet['ki'], ' ' . $string)), MB_CASE_TITLE, 'utf-8');
    }
}