Description:
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The warning message is only generated when display_errors = off

Tested using Apache 2.2.13 (win32)

Reproduce code:
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echo htmlspecialchars(chr(224), ENT_COMPAT, 'utf-8');

Expected result:
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PHP Warning:  htmlentities(): Invalid multibyte sequence in argument in [...] on line [...]

Actual result:
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display_errors = Off   ->   The error message is logged but not displayed (correct)
display_errors = On    ->   No error message is neither displayed nor logged (buggy?)

Obviously assuming log_errors = On.

With a custom error handler and display_errors = off the handler is called and the error informations are correct.

If display_errors = on the custom error handler isn't called.