How to determine that code works stably and predictably in various situations?

To determine that the code works stably and predictably in various situations, several approaches are used:

• Automated testing: covering the code with unit tests, integration tests, and functional tests helps verify behavior in different scenarios.
• Error and exception handling: the code should correctly handle unexpected situations without crashing.
• Logging and monitoring: collecting information about the application's operation in real-time helps identify instabilities.
• Testing on boundary values and with different input data: checking that the code works correctly with extreme and incorrect data.

For example, in PHP, you can write unit tests with PHPUnit that test functions with different parameters and expect certain results.