PHP Multiple header() calls causing http error 500

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PHP

I wrote a script that is intended to return multiple headers with information required by the calling scripts. It works fine on my server, which runs PHP 5.2.17, but on my client's server, which runs 5.3.6 it gives http error 500. I have narrowed it down to definitely being caused by a second call to header(). I see in the documentation:

4.4.2 and 5.1.2: This function now prevents more than one header to be sent at once as a protection against header injection attacks.

But if that's the case, is there an environment variable that turns this on and that's why it works on my server, running 5.2.17? Is there any way around it? I've tried putting all three headers I need in one header() call using a "\n" to add a carriage return between them, but that results in them not being sent at all on my server.

Any help is appreciated!

David