What do I do if no one admits me to a PhD position?
I think that everyone who is applying for PhD programs should consider their backup options. Admissions are not a guaranteed thing (unless you have a truly exceptional record), and even if you get in many people decide that a PhD is not right for them partway through. Identifying a backup plan is an important insurance policy to have.

This is especially important if you are applying during your last year of undergrad. If your applications don't go as well as you hoped, finding a job or other opportunity starting in April can be an impossible task.

If you are sure that you would still want to get a PhD even after a disappointing application round, make sure to research 1- to 2-year opportunities that can strengthen your resume. Research jobs, RA positions, masters programs, some industry positions, etc. are all things to look at, though availability may depend on your field and situation. If possible, you should also ask your adviser if they know of any opportunities or can put you in contact with anyone who might. 

Depending on the culture/positions, I would even consider applying to some along-side graduate school applications if you are unsure how they will go. I worked at a PhD stepping-stone-type job for a couple of years before applying, and we regularly interviewed applicants who were also applying to PhD programs. Many places won't be so understanding, so be careful with this, but we were fine with that situation.