RFC 822 specifies the date string in chapter 5.1 as

  date =  1*2DIGIT month 2DIGIT

2DIGIT is defined to be "exactly <n> occurrences  of (element). Thus 2DIGIT is a 2-digit number" [chapter 2.6].

PHP however returns the day value as one digit only, e.g.:

  Fri,  8 Mar 2002 10:33:37 +0000

This might seem to be a minor issue but actually makes date("r") or gmdate("r") unusable with software that expects two digit days. The Apache server is probably the most important one: creating a conditional request like 

  "If-Modified-Since: " . gmdate("r",$lastmtime)

will for example not work. It will not fail, but Apache won't recognize this date and always return the file, modified or not.

(And yes, Apache's behaviour should be modified, too, making it less picky)