Which line break in php mail header, \r\n or \n?
The CRLF \r\n, should be used according to the php documentation. Also, to conform to the RFC 2822 spec lines must be delimited by the carriage return character, CR \r immediately followed by the line feed, LF \n.
Since \r\n is native to Windows platforms and \n to Unix, you can use the PHP_EOLDocs constant on Windows, which is the appropriate new line character for the platform the script is currently running on.
Just in case a search engine picks this up and saves someone else the frustration I went through: here's an additional curiousity.
On php 5.2x on Linux, I had \r\n on my email headers in php mail(), after an upgrade to php 5.3.3, the formatting and sending mysteriously failed. Removing the \r fixed the script (after examining many many other possibilities).
As stated above, \r\n is what you should use according to the RFC, but this breaks your headers on several mail systems (f.i. Outlook 2003). Even though \n is not the 'proper' line break to use, in my experience it works correctly on all mail systems I've encountered so far. Because of this, I always use just \n.