This may or may not be true. Professors generally have different projects with different deadlines, levels of funding, and different stages of a particular project already completed. For e.g., a project may already have most of the fundamental work done by previous PhD students and all that remains is the application of the process/code/device, which is more appropriate for a MS student. Additionally, the estimated cost of an MS student is substantially less than a PhD student. This may make it more likely for you to be offered a position.
By saying that you want to do a PhD when you really want an MS will put the professor in the wrong frame of mind. If the professor really thinks you are going to be around for 5 years or so, he or she might give you something new and fundamental for your PhD project. And if you decide to stop with just the masters, you might not have enough material to write anything conclusive. This will put you and your advisor in very difficult positions.
So if you want an MS, apply for an MS - you can indicate the possibility of staying for a PhD in your application or statement of purpose.