Use of "empty character constant?"

Hi all, I'm a C newbie and just started experimenting with the PHP internals a couple weeks ago... A few days ago I noticed assignment/comparison of char variables to '' (nothing between single quotes) in, for example, the is_numeric_string function (zend_operators.h), snprintf.c, spprintf.c, etc. I don't understand what it's doing. :-/ The only references I can find are about "empty character constant" compiler errors, which is what happens if I try to use '' in the same way (VC++ Express). Though PHP compiles fine with these "constructs," so something must be special there? Sorry to bother the list with this if it's a general C thing that should be known and not PHP-code specific. But I've tried to figure it out on my own (which I have with other stuff so far :-)) and can't, so I'd appreciate it if someone can clue me in! Thanks, Matt