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.
