My guess would be false is returned by a non Apache module version of PHP (seems I'm right too:
Return Values
An associative array of all the HTTP headers in the current request, or FALSE on failure.
😉 ).... but why you'd use them I'm not sure...
Obvioulsy the safe answer is:
Note: You can also get at the value of the common CGI variables by reading them from the environment, which works whether or not you are using PHP as an Apache module. Use phpinfo() to see a list of all of the available environment variables.