Why might mbstring.func_overload not work in php 7.3?

Question

PHP

php works as an apache module.
I set mbstring.func_overload in httpd.conf to 2.
Check:

var_dump(phpversion());
var_dump(ini_get('default_charset'));
var_dump(ini_get('mbstring.func_overload'));

$str = 'Тестовая строка';

var_dump(mb_substr($str, 0, 6));

var_dump(substr($str, 0, 6));

It seems ok, it showed up. But, in different versions of php, the functions work as if the overload does not work at all.
This is the output in 7.1.33:

string(6) "7.1.33"
string(5) "UTF-8"
string(1) "2"
string(12) "Тестов"
string(12) "Тестов"

This is the output in 7.3.12:

string(6) "7.3.12"
string(5) "UTF-8"
string(1) "2"
string(12) "Тестов"
string(6) "Тес"

Answer 1

Warning
This functionality has been DEPRECATED as of PHP 7.2.0 and its use is highly discouraged.
Here it is in 7.3 and does not work.