I've only just now noticed this question, after somebody asked about it on Twitter.

SIGBABY is indeed in an inside joke. Back in 2001, we discussed most PHP related things on the #php.pecl channel on IRC. Wez Furlong's kid had just been born, and Wez would often sign off for a bit with "SIGBABY" - as in indication that he needed some family time with the little one. As this happened so much, I added SIGBABY to pcntl. I had no idea it was still in there though!