Difference of a PhD program for students entering with BS and MS Students who have a BS can apply for PhD programs like students with a Master's degree (in the US). I was thinking why should students with only Bachelor degrees be admitted while many with Masters degree apply too. I thought the program may differ for these two admitted students. So, my question is basically this: 

Is there any difference between the PhD program that an admitted student with a BS starts, and the PhD program that a student with a Master's degree goes through? (assuming they are admitted to the same program at the same university)
PhD programs in the US typically have coursework requirements in addition to the PhD dissertation, and depending on the program, these course requirements can be quite significant (as much as 50 credit hours or 2-3 years of course work.)  A student who enters the PhD program after an MS will often be given credit for much of these course work requirements based on their previous MS degree.  However, it is often the case that students are required to "make up" particular courses that they have not previously taken.   

Most programs have minimum credit hour requirements (total credit hours including courses and dissertation credits.)  A fairly common rule is "60 credits past an MS degree or 90 credit hours past a BS degree."  

These rules vary a lot between universities and even within the same university, so it's important to understand the local rules that will apply to you when you apply to enter a PhD program.