Description:
------------
the following code works as expected

php7.2 -r "echo \mb_stripos('Hello', 'e', 0, '8bit');"
> 1

in 7.3, however, a error is thrown

php7.3 -r "echo \mb_strpos('Hello', 'e', 0, '8bit');"
> php error

alternatively, mb_stripos does not write any output

php7.3 -r "echo \mb_stripos('Hello', 'e', 0, '8bit');"
> empty result

the following code works with iso-8859-1

php7.3 -r "echo \mb_stripos('Hello', 'e', 0, 'iso-8859-1');"


php7.3 -r "print_r(mb_list_encodings());" contains 8bit and iso-8859-1

Array
(
    [0] => pass
    [1] => auto
    [2] => wchar
    [3] => byte2be
    [4] => byte2le
    [5] => byte4be
    [6] => byte4le
    [7] => BASE64
    [8] => UUENCODE
    [9] => HTML-ENTITIES
    [10] => Quoted-Printable
    [11] => 7bit
    [12] => 8bit
    [13] => UCS-4
    [14] => UCS-4BE

7.2/php.ini and 7.3/php.ini are identical


PHP-Version

PHP 7.3.0RC3 (cli) (built: Oct 15 2018 12:05:08) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.3.0-dev, Copyright (c) 1998-2018 Zend Technologies
    with Zend OPcache v7.3.0RC3, Copyright (c) 1999-2018, by Zend Technologies
    with Xdebug v2.7.0beta1, Copyright (c) 2002-2018, by Derick Rethans

Test script:
---------------
echo \mb_strpos('Hello', 'e', 0, '8bit');

Expected result:
----------------
1

Actual result:
--------------
PHP Warning:  mb_strpos(): Unknown encoding or conversion error in Command line code on line 1
PHP Stack trace:
PHP   1. {main}() Command line code:0
PHP   2. mb_strpos() Command line code:1

Warning: mb_strpos(): Unknown encoding or conversion error in Command line code on line 1

Call Stack:
    0.0008     406848   1. {main}() Command line code:0
    0.0008     406848   2. mb_strpos() Command line code:1