Makefile config

I'm editing a Makefile, and I came across this problem:

Code:
COMMENT=        PHP Scripting Language
COMMENT?=       Version 2.2.x of Apache web server with ${WITH_MPM:L} MPM.

The first line is from php5/Makefile, and the second line is from www/apache22/Makefile. Why does the second line have a "?" after COMMENT?
 
?= indicates, that if variable is not defined, Makefile will define it

It's like using CC?=gcc
if CC is not defined (usually it's defined to system default compiler aka cc)


I find it weird to have comment?= in Makefile
 
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