Here you find the weather conditions and outlook for Phnom Penh constantly updated. Keep in mind when the forecast predicts rain, it is usually a heavy thunderstorm of about one hour in late afternoon, while the rest of the day is sunny and dry.
It almost never rains all day in Phnom Penh, such a pattern would be exceptional. However, on the south coast it can be raining days on end during the rainy season.
Livability surveys (e.g. by The Economist) regularly put Phnom Penh in the bottom 10 of "Livable Cities". Well, I say this: these survey guys are sitting overseas just collecting data, but have never actually been here, let alone live. This is for them: