There is currently no way for custom error handler function to determine, if expression which generate any particular error was called using @ operator or not.
 For example there will be no difference between:

$fp = fopen('unavailable_file','r');
 and 
$fp = @fopen('unavailable_file','r');

 Warning will be thrown in both cases, while it must be only thrown into first case, as requested by programmer. So there must be some way to distinguish these cases. 
 Normally error_reporting() function should return 0, being called from custom error handler in a case, when @ operator was used, but it returns error reporting level, which is by default. I think it is wrong behaviour and it should be improved in a way, described before.