To create your classified website in PHP language you need to either learn a PHP coding or to hire a PHP developer who could do a job for you. There are thousands of freelancers who are ready to do the job as per your desire and would prefer to do your work on priority basis. If you have decided to learn PHP code you should join either a PHP learning coaching or you can also self-learn the coding.
These days self-learning is quite popular just like a most popular website w3schools is a great resource for self-learning. Once you understand the basics of PHP you can start building your classified website, but that is not all because you will need to make a list of features that you want in your classifieds website and then you should start working on your classifieds website because without proper planning you cannot build a good classified website.
If you have decided neither option then you can choose an open source PHP based classifieds script which is also another great option because most popular classifieds scripts are available these days for free. But there are lots of hosted scripts as well which are quite popular and effective way to make your classified website. The major benefit of hosted classified script is that it offers you not only a script but also maintenance, support, and hosting.
You can also choose to get a free to download classifieds script which is available to self-hosting only. You can freely download the classifieds script and then upload and setup on your hosting server which will only cost you a hosting and maintenance cost. You can hire a developer who could help you in installing your script and you can ask him for an annual maintenance support which will not only save lots of your time but also will help you run your website smoothly.