The default configuration of PHP on most servers is setup so that the server will supply information about the PHP version being run. There's no point to broadcasting this information. While letting this be available is not a serious security hole, why tempt fate since it's so easy to turn this off?

You can turn this off by finding your php.ini file (the location will vary, but /etc is a popular place for this to be) and editing it. You're looking for the line that says:

expose_php = On

and you'll want to change the On to Off. Make this change, save the file, and restart your server and it'll no longer make the PHP version information available.