Convert unicode string to camel case

convert_to_camelcase.php

<?php

function substring(string $string, int $start, int $length = null): string
{
    return mb_substr($string, $start, $length, 'UTF-8');
}

function lower(string $string): string
{
    return mb_strtolower($string, 'UTF-8');
}

function lower_first(string $string): string
{
    return lower(substring($string, 0, 1)) . substring($string, 1);
}

function capitalize(string $string): string
{
    return mb_convert_case($string, MB_CASE_TITLE, 'UTF-8');
}

function convert_to_camel_case(string $string, bool $shouldLowerFirst = true)
{
    preg_match_all('~\p{Lu}?\p{Ll}+|\p{Lu}+(?=\p{Lu}\p{Ll})|\p{Lu}+|\p{N}+~u', $string, $matches);
    $string = implode('', array_map('capitalize', $matches[0]));

    return $shouldLowerFirst ? lower_first($string) : $string;
}

var_dump(convert_to_camel_case('safeHTML'));
var_dump(convert_to_camel_case('safe_html'));
var_dump(convert_to_camel_case('safe_HTML'));
var_dump(convert_to_camel_case('safe_Html'));
var_dump(convert_to_camel_case('escapeHTMLEntities'));
var_dump(convert_to_camel_case('escape_html_entities'));
var_dump(convert_to_camel_case('escape_HTML_entities'));
var_dump(convert_to_camel_case('escape_Html_entities'));
var_dump(convert_to_camel_case('escape___Html____entities'));
var_dump(convert_to_camel_case('XMLHttpRequest'));
var_dump(convert_to_camel_case('XmlHTTPRequest'));
var_dump(convert_to_camel_case('Příliš_ŽLUŤOUČKÝ_KŮŇ-úpěl  120ďábelských ód 10 krát za sebou'));