A PHP/MySQL design, like any other web / database designs cannot refer to the vacuum! A work of this type could be tiresome, and sooner or later, it would use up all your energy and motivation. You should look for a practical idea. Look around yourself; take a notebook and go around, to shops, libraries, ... Look for any thing you could model by database and cyberspace. The ideas would surround you very soon, and you will find it difficult to pick the best.
you are said that your interested is in sql(standard query language)
for that you must thourough with database management system.so database security is my suggestion to do the project.if you know how to secure a data then you will be the leader in software industry.