it never happens, because of the potential dual use of the muntions.
on its use in Gaza
If you are using them for their designated purpose and 'your gudiance system fails' and you take out civillians / civillian infrastructure I guess you are immune from a court case.
Under laws of warfare, white phosphorous, which is not considered a chemical weapon, is permissable for creating smoke screens and illuminating battlefields at night. What isn't permissable is targeting of civilians / instalations in civilian built up areas
http://law.bepress.com/cgi/viewcontent.cgi?article=9244&context=expresso
page 67 covers this under reference to article 2.1 and 2.2 of protocol 3 of The 1980 Convention on Prohibitions or Restrictions on the Use of Certain Conventional
Weapons which may be Deemed to be Excessively Injurious or to have Indiscriminate
Effects