$unicodeTable = array('<sup>1</sup>'=>'U+00B9', '<sup>2</sup>'=>'U+00B2', '<sup>3</sup>'=>'U+00B3', '<sup>4</sup>'=>'U+2074', '<sup>5</sup>'=>'U+2075');

function replace_unicode_escape_sequence($match) {
    return mb_convert_encoding(pack('H*', $match[1]), 'UTF-8', 'UCS-2BE');
}

function unicode_chr ($chr) {
    $x = explode("+", $chr);
    $str = "u".end($x);
    return preg_replace_callback('/\\u([0-9a-f]{4})/i', 'replace_unicode_escape_sequence', $str);
}

foreach($unicodeTable as $uKey=>$uValue){
    $text = str_replace($uKey, unicode_chr($uValue), $text);
}

x² or xⁿ

json_decode('"u00B3"') // for PHP 5.x

"u{00B2}" // for PHP 7.x