All of a sudden everyone is an expert on WP. 
In Gaza, even the Red Cross accepts that the intention is probably to use WP to create smoke rather than to deliberately injure; the Associated Press quotes the ICRC's Peter Herby as saying: "It's not very unusual to use phosphorus to create smoke or illuminate a target. We have no evidence to suggest it's being used in any other way."
WP smoke rounds are not classed as an incendiary weapon either. This is because the 1980 Geneva Protocol on Incendiary Weapons specifically does not cover "Munitions which may have incidental incendiary effects, such as illuminants, tracers, smoke or signaling systems."
… the case put forward by Human Rights Watch and others is not that WP is an incendiary, but that its use "violates the requirement under international humanitarian law to take all feasible precautions to avoid civilian injury and loss of life." This looks like a reference to a different treaty, the 1949 Geneva Protocol, which has a section on "General protection against effects of hostilities".
http://blog.wired.com/defense/2009/01/white-phosphoru.html