Should I get a master's before I get my Ph.D.?

My advice would be to just go into the phd program--most don't require a separate masters. You're better off using the time you would spend on the masters doing actual enviro work or activism in the field. or better yet, skip the phd. the world has plenty of academics, and needs more thoughtful activists.

you might even get the masters while on your way to the phd, depending on the program.

If you already have a masters degree, it will take significant time off your Ph.D. program. Many programs automatically give you a Masters on the way to Ph.D.--If you know for sure you want a Ph.D. just go right to that program, and the masters will appear one day.

whether or not you have a masters doesn't really matter, like everyone said, you will probably get one on the way to the phd. More importantly, you should have work experience (an actual job, not an internship or co-op) before you graduate with your phd. otherwise, you become an overeducated, inexperienced job candidate. The phd will put you in a pay scale that you don't really deserve. If you are looking to work in the university environment, then this doesn't apply as much. But private industry (engineering firms, environmental consulting firms, non-profit organizations) are not going to want to pay you the phd salary while you are climbimg the learning curve.

if you want to go into academia, get a PhD, if not, a masters will get you the same amount of money and opportunities in the private sector (for the most part)

please no! Unless you apply to a "joint" program that does both. If you know you want the PhD, then apply for one. Otherwise, you'll take longer and not have any easier time getting in (unless you continue at the same place).

and you'll get more funding opportunities for phd study.

Yup, PhD programs often will fund your tuition and give you a living stipend, whereas you'll have to pay for a Masters (much harder to find scholarships). Most PhD programs will offer a terminal masters to their students who decide not to pursue the PhD, so even if you change your mind, you'll have the MA and you'll have saved the two or so years of tuition you would have paid for a Masters program.

I think it depends on the subject. Most advanced will require a masters degree or what they call “an equivalent qualification”, if you have lots of work experience this would be an equivalent qualification. A masters is generally a good idea, it set me on the path to research rigor.