Writing good code, organized and commented is not only for php programming, but for all the good programming languages today we must make "clean code". A clean code is much more readable for a developer, which gives a huge margin of improvisation every time an intervention is necessary. Sometimes developers use single-letter variables just to speed things up, and in the end they end up with unreadable code and code that is too slow or too resource-intensive. An organized and clean code not only boosts the loading time of pages but also allows a much better maintainability of the code when it is necessary to modify or optimize it.
Avoid letting queries run in a loop
Create a table of data and add two separate queries, instead of letting them work in a loop ... In some situations, running the queries in a loop will bring faster results, but it will add to the load on your hosting server in most of the time and you will end up with a slow site from the first peaks of visits.
Avoid the use of closing tags
Some developers use closing tags to terminate PHP files whereas this is not required in PHP. Most WordPress files already ignore this bad practice, but novice developers still use it to keep spaces under control, believing it's a good practice. But the latest versions of PHP never highlight the use of closing tags in the final part of the "?>" Files. So avoid it.
Stop using MySQL functions
PHP is always delivered with many advanced features, including those for the handling of MySQL databases that are still quite old and not sufficiently updated. The best solution is to use the new "MySqli" functions with all advanced versions of PHP. The "MySqli" functions support the PDO implementation and make database access much easier than "MySql" functions. Thus, developers can produce high quality results using the "MySqli" functions with the latest versions of PHP.