Add missing core constants

Closes php#214.

Author: alexanderholman

spec/06-constants.md

@@ -109,11 +109,16 @@ Constant Name | Description
 `PHP_INT_MAX` | `int`; the maximum representable value for an integer.
 `PHP_INT_MIN` | `int`; the minimum representable value for an integer.
 `PHP_INT_SIZE` |  `int`; the number of bytes used to represent an integer.
+`PHP_FLOAT_DIG` | `int`; the number of decimal digits that can be rounded into a float and back without precision loss.
+`PHP_FLOAT_EPSILON` | `float`; the smallest representable positive number `x`, so that `x + 1.0 != 1.0`.
+`PHP_FLOAT_MIN` | `float`; the smallest representable normalized floating point number larger than zero.
+`PHP_FLOAT_MAX` | `float`; the largest representable floating point number.
 `PHP_MAJOR_VERSION` | `int`; the current PHP major version
 `PHP_MANDIR`  | `string`; the installation location of the manual pages.
 `PHP_MAXPATHLEN` |  `int`; the maximum length of a fully qualified filename supported by this build.
 `PHP_MINOR_VERSION` | `int`; the current PHP minor version.
 `PHP_OS` | `string`; the current operating system.
+`PHP_OS_FAMILY` | `string`; the operating system family PHP was built for. Either of 'Windows', 'BSD', 'Darwin', 'Solaris', 'Linux' or 'Unknown'.
 `PHP_PREFIX`  | `string`; the value to which "--prefix" was set when configured.
 `PHP_RELEASE_VERSION` | `int`; the current PHP release version.
 `PHP_ROUND_HALF_DOWN` | `int`; Round halves down.