this happens because there's a "rm -rf $(TARGET_DIR)/usr/lib/php" on line 298 in php.mk maybe it was rather meant to delete /usr/lib/php/build or sth?
a little more explaination: opcache is the only extension (i know of) that gets installed to /usr/lib/php, rather than being linked into the binary, so removing that directory also removes the extension
Should be fixed by http://patchwork.ozlabs.org/patch/473593/.
Fixed by http://git.buildroot.net/buildroot/commit/package/php?id=ec0404a9a426e866dff20bf11b33c06b90f9f85d.