Failed assertions on php-7.3 and php-7.4 versions

[peter279k]

When executing PHPUnit tests on php-7.3 and php-7.4 versions, it will output following error messages:

1) Str\StrTest::testModifiers
Failed asserting that Str\Str Object &0000000065f8b35f00000000357bdec6 (
    's' => 'önneki̇ş'
) matches expected 'önnekiş'.

When fixing this assertion, executing this PHPUnit tests again, it will be passed on php-7.3 and php-7.4 versions, but it will be failed on php-7.1 and php-7.2 versions.

And the failure message is as follows:

1) Str\StrTest::testModifiers
Failed asserting that Str\Str Object &0000000001d924ba000000003a686fd1 (
    's' => 'önnekiş'
) matches expected 'önneki̇ş'.

I think this problem is about internal mb_string extension and it's hard to be resolved.

@fe3dback, what do you think about this?