--enable-ext VS --with-ext

I'm reading the comments in the config.m4 generated by ext_skel and trying
to understand under what circumstances one would use --enable-ext and when
one would use --with-ext.

What constitutes "something external" ?




dnl If your extension references something external, use with:

dnl PHP_ARG_WITH(fmog, for fmog support,
dnl Make sure that the comment is aligned:
dnl [  --with-fmog             Include fmog support])

dnl Otherwise use enable:

dnl PHP_ARG_ENABLE(fmog, whether to enable fmog support,
dnl Make sure that the comment is aligned:
dnl [  --enable-fmog           Enable fmog support])