(PHP 4 >= 4.0.6, PHP 5, PHP 7, PHP 8)
mb_internal_encoding — Set/Get internal character encoding
Set/Get the internal character encoding
encoding
 encoding is the character encoding name 
 used for the HTTP input character encoding conversion, HTTP output 
 character encoding conversion, and the default character encoding 
 for string functions defined by the mbstring module.
 You should notice that the internal encoding is totally different from the one for multibyte regex.
 
 If encoding is set, then 
 Returns true  on success or false  on failure.
 In this case, the character encoding for multibyte regex is NOT changed.
 If encoding is omitted, then 
 the current character encoding name is returned.
 
 As of PHP 8.0.0, a ValueError  is thrown if the
 value of encoding is an invalid encoding.
 Prior to PHP 8.0.0, a E_WARNING  was emitted instead.
 
| Version | Description | 
|---|---|
| 8.0.0 | encodingis nullable now. | 
| 8.0.0 | Now throws a ValueError  if encodingis an invalid encoding.
 Previously aE_WARNING was emitted instead. | 
Example #1 mb_internal_encoding() example
<?php
/* Set internal character encoding to UTF-8 */
mb_internal_encoding("UTF-8");
/* Display current internal character encoding */
echo mb_internal_encoding();
?>
Especially when writing PHP scripts for use on different servers, it is a very good idea to explicitly set the internal encoding somewhere on top of every document served, e.g.
mb_internal_encoding("UTF-8");
This, in combination with mysql-statement "SET NAMES 'utf8'", will save a lot of debugging trouble. 
Also, use the multi-byte string functions instead of the ones you may be used to, e.g. mb_strlen() instead of strlen(), etc.all together 
<?php
// ------------------------------------------------------------ 
header('Content-Type: text/html; charset=UTF-8');
mb_internal_encoding('UTF-8'); 
mb_http_output('UTF-8'); 
mb_http_input('UTF-8'); 
mb_regex_encoding('UTF-8'); 
// ------------------------------------------------------------ 
?>Be aware that the strings in your source files must match the encoding you specify by mb_internal_encoding. It appears the Parser loads raw bytes from the file and refers to its internal encoding to determine their actual encoding.
To demonstrate, the following outputs as espected when the /source/ file is Latin-1 encoded:
<?php
 mb_internal_encoding("iso-8859-1");
 mb_http_output( "UTF-8" );
 ob_start("mb_output_handler");
 echo "???<br/>";
 ?>???
Now, a typical use of mb_internal_encoding is shown as follows. Make the change to "utf-8" but leave the /source/ file encoding unchanged:
<?php
 mb_internal_encoding("UTF-8");
 mb_http_output( "UTF-8" );
 ob_start("mb_output_handler");
 echo "???<br/>";
 ?>???
The output will just show the <br/> tag and no text.
Save the file as UTF-8 encoding and then the results will be as expected.In response to mortoray at ecircle-ag dot com:
The characters display fine as long as you set the Encoding to something more "Latin 1" compatible (i.e. US-ACSII, ISO-8859-1, ISO-8859-1, or Windows 1252). PHP.net auto-detects to UTF-8