convert camel to underscore

camelCaseHeheHihiTestOK
=> 
camel_case_hehe_hihi_test_ok

/**
* Converts a camel cased string to a snake cased string.
*/
function convertCamelCaseToUnderscore(string $string): string
{
  return mb_strtolower(preg_replace('/(?<!^)((?=[\p{Lu}][^\p{Lu}])|(?<![\p{Lu}])(?=[\p{Lu}]))/', '_', $string));
}