ResourceBundle::__construct() uses ures_open(), which performs validity checks on the given locale identifier. That's reasonable, as the fallback functionality only works with proper locale IDs (example: you pass "de_DE", but no such bundle exist, it will then use the bundle "de" if that exists).
With the fallback flag off (third ctor argument), ures_open is still used. This won't allow loading of data in invalid identifiers, such as some of ICU's built-in data (e.g. "supplementalData"). http://bugs.php.net/bug.php?id=54540 has a patch, but I wanted to run it by Hans-Peter and Stas again in case they want to discuss this. David
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