Weird behaviour when changing the different php.ini settings to influence the default mb_internal_encoding() encoding.

php-encoding-test.php

<?php 
echo "\n\nPHP: " . phpversion()."\n";
echo "Default:                                         " . mb_internal_encoding()."\n";
echo "Old value of default_charset:                    ".ini_set("default_charset", "ASCII")."\n";
echo "when default_charset is set to ASCII:            " . mb_internal_encoding()."\n";
echo "Old value of internal_encoding:                  ".ini_set("internal_encoding", "ASCII")."\n";
echo "when internal_encoding is set to ASCII:          " . mb_internal_encoding()."\n";
echo "Old value of mbstring.mb_internal_encoding:      ". ini_set("mbstring.internal_encoding", "ASCII")."\n";
echo "when mbstring.internal_encoding is set to ASCII: " . mb_internal_encoding()."\n";


/** Output:

PHP: 7.0.21-1~ubuntu16.04.1+deb.sury.org+1
Default:                                         UTF-8
Old value of default_charset:                    UTF-8
when default_charset is set to ASCII:            UTF-8
Old value of internal_encoding:
when internal_encoding is set to ASCII:          UTF-8
Old value of mbstring.mb_internal_encoding:
when mbstring.internal_encoding is set to ASCII: ASCII
*/

/** Bash version for easy c&p
vagrant@atmo:~$ php -r '
echo "\n\nPHP: " . phpversion()."\n";
echo "Default:                                         " . mb_internal_encoding()."\n";
echo "Old value of default_charset:                    ".ini_set("default_charset", "ASCII")."\n";
echo "when default_charset is set to ASCII:            " . mb_internal_encoding()."\n";
echo "Old value of internal_encoding:                  ".ini_set("internal_encoding", "ASCII")."\n";
echo "when internal_encoding is set to ASCII:          " . mb_internal_encoding()."\n";
echo "Old value of mbstring.mb_internal_encoding:      ". ini_set("mbstring.internal_encoding", "ASCII")."\n";
echo "when mbstring.internal_encoding is set to ASCII: " . mb_internal_encoding()."\n";
*/
?>