PHP is a widely used programming language with a low barrier to enter, that means anyone can say they are a PHP programmer, however, if you are looking for the best of the best or the top echelon of programmers for either a project or full time hire, here are some tips on how to attract and interview candidates. Preparing well will help you sort through candidates quickly but if this is an endeavor that you feel will take up too much in-house time and money, consider hiring a reputable recruiter.
1.) Clearly convey the project/position and what this position will accomplish. This is the time to be extra thorough to weed out
the more entry-level programmers. Be sure to mention at this time, all other languages and technologies you expect this position to know and who will be designing the front-end.
2.) Now you will receive a moderate amount of applicants. Prep them by asking the best questions in the interview. Here are some questions from Toptal that may help you ask some technical aspects of the position. Aside from reading code, here are some questions to see if they understand PHP code. https://www.toptal.com/php#hiring-guide?x=q
3.) All-in-all this is a two part interview process. One part is to be sure they not only grasp but can handle the work load you are expecting from this position. The second part is that they will fit well with your style and your team.
Since there are so many PHP programmers from entry-level to advanced, you will have an overwhelming response to the job posting. However, you have some options, you can prepare well for the temporary increased workload or hire a recruiter with top PHP connections. Good luck!