How a PHP request actually runs
What happens between nginx and your PHP code: PHP-FPM, OPcache and the share-nothing model - and why they explain most PHP performance and scaling problems.
Backend
When the numbers don't agree, trust is the first thing to go. Backend work as correctness the business can rely on - data, money and rules that hold up.
Handling money in software: why floats will eventually cost you
Why floating-point numbers can't hold money, how to store and round amounts correctly, and how PostgreSQL, MySQL, Java, Python, JavaScript, Go and PHP handle it.