After I was doing some GLSA updating, PHP stopped working for me. 

php: error while loading shared libraries: libhistory.so.4: 
cannot open shared object file: No such file or directory


On the Gentoo forums, I found the solution: 

revdep-rebuild

This command rebuilds all packages broken by an upgrade.

Code: 
shake / # revdep-rebuild

Checking reverse dependencies...
Packages containing binaries and libraries broken by any package update,
will be recompiled.

Collecting system binaries and libraries... done.
  (/root/.revdep-rebuild.1_files)

Collecting complete LD_LIBRARY_PATH... done.
  (/root/.revdep-rebuild.2_ldpath)

Checking dynamic linking consistency...
  broken /usr/bin/php (requires libhistory.so.4)
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