  The phase diagram of LaFeAs$_{1-x}$P$_x$O system has been extensively studied
through hole- and electron-doping as well as As/P-substitution. It has been
revealed that there are three different superconducting phases with different
Fermi surface (FS) topologies and thus with possibly different pairing glues.
One of them is well understood as spin fluctuation-mediated superconductivity
within a FS nesting scenario. Another one with the FSs in a bad nesting
condition must be explained in a different context such as orbital or spin
fluctuation in strongly correlated electronic system. In both phases,
$T$-linear resistivity was commonly observed when the superconducting
transition temperature $T_{\rm c}$ becomes the highest value, indicating that
the strength of bosonic fluctuation determines $T_{\rm c}$. In the last
superconducting phase, the nesting condition of FSs and the related bosonic
fluctuation are moderate. Variety of phase diagram characterizes the multiple
orbital nature of the iron-based superconductors which are just near the
boundary between weak and strong correlation regimes.
