Is it harder to get into a Ph.D. program if you attend multiple undergrad institutions?
Yes. Absolutely.
Not directly, but think of it this way: in order to be admitted to a Ph.D. program, you need (more or less in the order of importance) (1) research experience, (2) great letters of recommendation, (3) high GPA, and (4) high GRE scores.
If you keep transferring from school to school in undergrad, how can you get (1) and (2)?
Note that Ph.D. admission is extremely competitive.
@Pentaprism that is a good point, thank you.
I think it depends a lot on what you mean by “multiple”. If you transferred from one school to another, you probably won’t have too difficult a time. If you’ve been to 5 different schools in as many years, that’s going to be a harder sell.