When having a file saved in ISO-8859-1 and using the command
echo "test: ".htmlspecialchars("äöü");
The return will be only "test: ".
This is because the standard charset for htmlspecialchars changed to UTF-8 in PHP5.4. You need to explicitly set the charset:
echo "test: ".htmlspecialchars("äöü", ENT_COMPAT | ENT_HTML401, 'ISO-8859-1');
Are there any other functions in PHP5.4, that will not work properly anymore, if you do not set the charset?
htmlentities seems to be another function that was changed: http://de3.php.net/manual/de/migration54.other.php
The migration guide from PHP 5.2->5.3 does not give any more functions that changed the default charset: http://de3.php.net/manual/de/migration53.php
So probably it is only htmlspecialchars() and htmlentities
Anyway, I think those two should definitely go to the "Backward Incompatible Changes"-list http://de3.php.net/manual/de/migration54.incompatible.php