Not sure if it makes sense to change only one header call. If you search in piwik for @header you will dozens of such calls...

if there are so many, it must be factorized. For example, in the exception handle method to put it on all exceptions.

there's already a function that we should reuse https://github.com/piwik/piwik/blob/master/core/Common.php#L1041

This is legacy code, it will be removed eventually when output formatting systems are further refactored.

Ok, so if you think it would be better to remove @ in front of each header(), I can look for all header() calls that do not uses Common, put the test in Common and redo a pull-request.

@kylekatarnls sounds like a good plan, @diosmosis @tsteur any thoughts?

Best would be to solve the actual problem meaning: Why are there already headers sent before at all? And when can headers be already sent and when not? Probably needs a deeper look. Sounds like a component from a deeper layer maybe already sends a header that is not supposed to send one. But didn't have a detailed look.

In general +1 for removing @ . It is bad practice to use it.

Here is the list of all @Header() in the code and one @$_SERVER['...']
https://github.com/kylekatarnls/piwik/compare/cleanup-arobases
I can send it if you want to use it while inspect the code deeper.

sendHeader does not put the @ in front of header call - your change to replace all header() calls by sendHeader() looks good!

Maybe you can issue new pull request for that change @kylekatarnls ?