What happens when `define()` is called with a constant that already exists?

When define() is called with a constant that already exists, it triggers a warning and does not redefine the constant. In PHP, constants are immutable once they are set. This means that attempting to redefine an existing constant is not allowed, preserving the original value and preventing any unintended side effects.

This behavior helps ensure that constants provide a reliable value throughout the execution of a script, as constants are intended to represent fixed values that do not change. If a programmer mistakenly tries to redefine a constant, the warning serves as an alert that there might be an oversight in the code, prompting them to check the definition and usage of the constant, maintaining code quality and reducing potential bugs.