PHP strtolower converts UTF-8 wrong

Converting UTF-8 strings produces invalid characters, when a UTF-8 locale is set, otherwise they are just ignored, wich is fine.

<?php
    $t = 'ö';
    $c = strtolower($t);
    print(implode(unpack("H*", $t)) . "\t" . implode(unpack("H*", $c)) . "\n");
?>

LC_ALL="C" php test.php 
c3b6    c3b6
LC_ALL="en_US.UTF-8" php test.php 
c3b6    e3b6

In this case, it "converts" an already lowercase character, but it's the same for uppercase Umlauts, Ö(c396) will end up as e396 instead of c3b6.

It works on Linux, i do not yet know if it's a problem in Illumos or PHP.

The mb_strtolower($keywords, 'UTF-8') way works however.

Tested with:
PHP 5.3.29 (cli) (built: Jan 29 2016 19:08:29)
PHP 7.0.27 (cli) (built: Apr 8 2018 20:20:23) ( NTS )

Start by reproducing the issue with the provided test.php snippet under C and en_US.UTF-8 locales, comparing strtolower with mb_strtolower. Check whether the behavior is specific to PHP or Illumos, then identify the appropriate upstream or pkgsrc component. Done means the UTF-8 result is correct or the issue is documented with its confirmed source and scope.