PHP string to subscript and superscript string

SubSupText.php

<?php
/**** example usage ****
print SubSupText::replaceSubSupHtml("H<sub>2</sub>O") . PHP_EOL; //H₂O
print SubSupText::replaceSubSupHtml("42km<sup>2</sup>") . PHP_EOL; //42km²
print SubSupText::replaceSubSupHtml("2<sup>th</sup> chapter") . PHP_EOL; //2ᵗʰ chapter
*/

class SubSupText{
    const superscriptMap = [
        '0' => '⁰', '1' => '¹', '2' => '²', '3' => '³', '4' => '⁴', '5' => '⁵',
        '6' => '⁶', '7' => '⁷', '8' => '⁸', '9' => '⁹', 'a' => 'ᵃ', 'b' => 'ᵇ',
        'c' => 'ᶜ', 'd' => 'ᵈ', 'e' => 'ᵉ', 'f' => 'ᶠ', 'g' => 'ᵍ', 'h' => 'ʰ',
        'i' => 'ⁱ', 'j' => 'ʲ', 'k' => 'ᵏ', 'l' => 'ˡ', 'm' => 'ᵐ', 'n' => 'ⁿ',
        'o' => 'ᵒ', 'p' => 'ᵖ', 'q' => 'q', 'r' => 'ʳ', 's' => 'ˢ', 't' => 'ᵗ',
        'u' => 'ᵘ', 'v' => 'ᵛ', 'w' => 'ʷ', 'x' => 'ˣ', 'y' => 'ʸ', 'z' => 'ᶻ',
    ];
    const subscriptMap = [
        '0' => '₀', '1' => '₁', '2' => '₂', '3' => '₃', '4' => '₄', '5' => '₅',
        '6' => '₆', '7' => '₇', '8' => '₈', '9' => '₉', 'a' => 'ₐ', 'e' => 'ₑ', 
        'h' => 'ₕ', 'i' => 'ᵢ', 'j' => 'ⱼ', 'k' => 'ₖ', 'l' => 'ₗ', 'm' => 'ₘ', 
        'n' => 'ₙ', 'o' => 'ₒ', 'p' => 'ₚ', 'r' => 'ᵣ', 's' => 'ₛ', 't' => 'ₜ',
        'u' => 'ᵤ', 'v' => 'ᵥ', 'x' => 'ₓ', 'y' => 'ᵧ', 'z' => '𝓏',
    ];

    public static function toSuperscript(string $str): string{
        $outputString = '';
        $length = mb_strlen($str, 'UTF-8');

        for ($i = 0; $i < $length; $i++) {
            $char = mb_substr($str, $i, 1, 'UTF-8');
            $outputString .= isset(self::superscriptMap[$char]) ? self::superscriptMap[$char] : $char;
        }

        return $outputString;
    }

    public static function toSubscript(string $str): string{
        $outputString = '';
        $length = mb_strlen($str, 'UTF-8');

        for ($i = 0; $i < $length; $i++) {
            $char = mb_substr($str, $i, 1, 'UTF-8');
            $outputString .= isset(self::subscriptMap[$char]) ? self::subscriptMap[$char] : $char;
        }

        return $outputString;
    }

    public static function replaceSubSupHtml(string $htmlString): string {
        $supPattern = '/<sup>(.*?)<\/sup>/s';
        $htmlString = preg_replace_callback($supPattern, function ($matches) {
            return self::toSuperscript($matches[1]);
        }, $htmlString);

        $subPattern = '/<sub>(.*?)<\/sub>/s';
        return preg_replace_callback($subPattern, function ($matches) {
            return self::toSubscript($matches[1]);
        }, $htmlString);
    }
}

not complete at all just a quick hack to fix some html strings to be unicode only.
might be useful to someone.
if you come up with any improvements please write a comment.