""Fully funded"" is doing a lot of heavy lifting in PhD admissions, and I have learned not to trust it until I have seen it in writing. Sometimes it means five guaranteed years. Sometimes it means we will figure it out, which is not funding. That is optimism.
The real question is never whether funding exists. It is whether it is guaranteed, for how long, and on paper. A program that funds everyone for five years and says so publicly is a different universe from one where your support depends on your advisor's grant renewing next spring. Ask the uncomfortable question directly: how many years are guaranteed, and what happens if my advisor loses funding? Then watch how fast they answer. A quick, specific answer is a good sign. Hesitation is data. Awkward silence is louder data.
Fully funded should also mean tuition covered, and mostly it does. Most reputable US PhD programs waive tuition on top of the stipend. Fees are where it gets sneaky. Student fees, health fees, technology fees, a gym fee for a gym you will never visit. Some programs waive them. Others quietly leave you paying a few thousand a year out of your stipend. A couple thousand dollars is a serious pay cut on a grad budget, and it never shows up in the headline number.
Summer is the other trap. A nine-month stipend with no summer support means you are scrambling for teaching or going unpaid for a quarter of the year. Programs with twelve-month funding are worth real money over five years, even when the headline looks similar.
This is why a real PhD funding comparison lines programs up side by side: stipend, guaranteed years, summer money, fees, insurance. The best funded PhD programs database tool is the one that shows all of it in one place instead of making you assemble it from departmental websites and rumor. Filter for guaranteed funding first and stipend amount second. phdstipend.fyi lets you compare by guaranteed funding years, not just the big number, which is exactly the right order of operations.