difference between --enable-mbstring and --enable-mbstring=shared

Difference is that one use dynamic linking, the other static linking.
Theoretically it shouldn't matter if you use either. The only difference
should be when you upgrade your mbstring library. When using static
linking (ie. not shared), PHP still use the OLD library, because the
compiler "built" it in the PHP "executable", while dynamic linking will
see PHP using the new updated library.