Hi @smalyshev, sorry I didn't answer earlier, I've been busy with work.

RedHat created its own icu.pc (which contains the same data as if icu-uc.pc and icu-i18n.pc were combined together).
There was a discussion some time ago in the ICU project to create a file named icu.pc for compatibility with RH, but this was rejected since Fedora/RH later decided to drop that file and use the regular .pc files provided by ICU.

See also https://bugzilla.redhat.com/show_bug.cgi?id=460168 and http://bugs.icu-project.org/trac/ticket/9478 for details.

Anyway, even on RH/CentOS, PHP should still continue to build since the code falls back to the old behaviour (using pkg-config) in case icu-io.pc cannot be found.