1. Do not use relative paths, instead define a ROOT path
2. Don’t use require , include , require_once or include_once
3. Maintain debugging environment in your application
4. Propagate status messages via session
5. Make your functions flexible
6. Omit the closing php tag if it is the last thing in a script
7. Collect all output at one place, and output at one shot to the browser
8. Send correct mime types via header when outputting non-html content
9. Set the correct character encoding for a mysql connection
10. Use htmlentities with the correct characterset option
11. Do not gzip output in your application , make apache do that
12. Use json_encode when echoing JavaScript code from php
13. Check if directory is writable before writing any files
15. Don’t check submit button value to check form submission
16. Use static variables in function where they always have same value
17. Don’t use the $_SESSION variable directly
18. Wrap utility helper functions into a class
19. Bunch of silly tips
20. Process arrays quickly with array_map
21. Validate data with php filters
22. Force type checking
23. Write Php errors to file using set_error_handler()
24. Handle large arrays carefully
25. Use a single database connection, throughout the script
26. Avoid direct SQL query, abstract it
28. Store sessions in database
29. Avoid using globals
30. Use base url in head tag
31. Manage error reporting
32. Be aware of platform architecture
33. Dont rely on set_time_limit too much
34. Make a portable function for executing shell commands
35. Localize your application
36. Use a profiler like xdebug if you need to
37. Use plenty of external libraries
38. Have a look at phpbench for some micro-optimisation stats
39. Use an MVC framework
40. Read the comments on the php documentation website
41. Read open-source code
42. Develop on Linux
Resources
https://www.binarytides.com/35-techniques-to-enhance-your-php-code/