Description:
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When the first output happens, there is a record of that in PHP core.

Then, the error message for the header() that follows output can tell the programmer exactly where in their mass of code the output started.

While not quite the same scope, in a large code base, even grepping for session_start() is going to have a lot of hits.

Would it be hard to notate where session_start() got called, and include in the error message...