src/Russian/RussianLanguage.php
public static function choosePrepositionByFirstLetter($word, $prepositionWithVowel, $preposition)
    {
        if (in_array(S::lower(S::slice($word, 0, 1)), ['а', 'о', 'и', 'у', 'э'], true)) {
            return $prepositionWithVowel;
        } else {
            return $preposition;
        }
    }
public static function choosePrepositionByFirstLetter($word, $prepositionWithVowel, $preposition)
    {
        if (in_array(S::lower(S::slice($word, 0, 1)), ['а', 'о', 'и', 'у', 'э'], true)) {
            return $prepositionWithVowel;
        } else {
            return $preposition;
        }
    }
Выбор предлога по первой букве