Weirdness with PHP FreeBSD/OSX/Linux headers already sent

PHP List, Perhaps one of you might know the answer to this, because I certainly do not, nor do any of my coworkers. The "problem" is that scripts on our OSX 10.4 (our local development machines) and RedHat/Debian linux servers do not throw the "headers already sent" message. I could have an entire page of html, php code, whatever I wish and call session_start() at the bottom of the page and everything would be fine. No errors on screen, in error_log, or anywhere else. I know this is invalid and so do most webservers/environments because everywhere else I have ever coded I see the "headers already sent, output started at" message. I have used Windows, Linux, and FreeBSD versions of PHP with Apache 1.3 and 2 and had the error print out as expected. I want to know if any of you know the reason why this is happening. I would prefer the strict error messages to help keep everyones coding in check. Unfortunately I cannot for the life of me seem to find any configuration settings which point to this. Any pointers or help would be greatly appreciated. Thank you in advance!