user@localhost ~ $ emerge -uDNpvt php

These are the packages that would be merged, in reverse order:

Calculating dependencies /
!!! All ebuilds that could satisfy "x11-libs/libXpm" have been masked.
!!! One of the following masked packages is required to complete your request:
- x11-libs/libXpm-3.5.5 (masked by: ~amd64 keyword)
- x11-libs/libXpm-3.5.4.2 (masked by: ~amd64 keyword)

For more information, see MASKED PACKAGES section in the emerge man page or
refer to the Gentoo Handbook.
(dependency required by "dev-lang/php-5.1.4" [ebuild])