What if you can't publish in very high impact journal or top conference during your PhD?
The goal of a Ph.D. is to learn to be an effective scientific researcher.  If you accomplish that during your Ph.D., then you will have been successful.

Moreover, early in one's career, high-rank publications don't actually correlate strongly with being an effective researcher.  They're much more about happening to be in the right research group at the right time with the right guidance.

Now, if things line up for you and it happens that your Ph.D. also generates some high-rank publications during that time, it will make your next stage easier, but it is by no means necessary.  

Remember:


Most post-Ph.D. careers do not require high-rank publications.  Highly focused fields, industry, teaching-centric colleges, and many other areas will care about the solidity of your work much more than its glamour.
Interesting work often takes a long time to mature, and important work often shows up in high-rank venues only after it has been incubating in multiple publications in "bread and butter" venues over the course of years.
If you do want a career that will be boosted by high-rank publications, one or more postdocs are an excellent period to obtain such. They also let you demonstrate that you are the important factor, and that it's not just that you happened to get lucky in your Ph.D. subject and advisor.


In short: stop worrying and instead just focus on doing something interesting and worthwhile with the work you're doing now.