Note on page for header() needs clarification.

On the manual page for header(), in the Notes section, the second note starts "You can use output buffering to get around this problem, ...".

What is "this problem"? The Wayback machine says it it refers to the paragraph right back up in the Description section about headers having to be sent before output is generated (the note was relocated during a redesign of the manual in 2007 without being edited).

Also, calling it a "problem" might be inaccurate. If it becomes a problem, then there are measures you can take to work around it, but by itself it's just an assertion that headers must precede the body.