As far as I can see, the only error than can realistically happen is that the input may be bad UTF-8. In which case, the function returns FALSE, not NULL, as is usual in the intl extension.

There's one code path in the function that indeed returns null (probably an oversight), but it's a path that should never be reached.

As for the exceptions, you can turn them on in intl PECLv3/PHP 5.5 (to use it in PHP 5.3, remove your current intl extension and install the PECL version with pecl install intl-beta):

$ php -d intl.use_exceptions=1 -r 'var_dump(normalizer_normalize("\x80", Normalizer::FORM_C));'
PHP Fatal error:  Uncaught exception 'IntlException' with message 'Error converting input string to UTF-16' in Command line code:1
Stack trace:
#0 Command line code(1): normalizer_normalize('?', 4)
#1 {main}
  thrown in Command line code on line 1
$ php -d intl.use_exceptions=1 -r 'var_dump(normalizer_normalize("", -99));'
PHP Fatal error:  Uncaught exception 'IntlException' with message 'normalizer_normalize: illegal normalization form' in Command line code:1
Stack trace:
#0 Command line code(1): normalizer_normalize('', -99)
#1 {main}
  thrown in Command line code on line 1

For older versions, you can check the errors with intl_get_error_code()/intl_get_error_message(). You can also make the errors throw notices/warnings/whatever:

$ php -d intl.error_level=E_WARNING -r 'var_dump(normalizer_normalize("", -99));'
PHP Warning:  normalizer_normalize(): normalizer_normalize: illegal normalization form in Command line code on line 1
bool(false)

Thanks for the quick reply.

So probably all that needs to be done here is to fix the documentation. For example, it was not at all clear to me that the way to get an error message was you use the mechanisms provded by the intl extension. Nor is the return FALSE documented.