Problems with PHP constants. The value of the PHP constant changed, wondering

Php

Problems with PHP constants. The value of the PHP constant cannot be changed, but the value of this code changes a bit. Please help me to analyze.
The code is as follows:
Define ("CONSTANT", "hello!", true);
echo CONSTANT. "
";
Define ("CONSTANT", "What is you doing?");
echo CONSTANT. "
";
Echo Constant;
The page output is like this:
Hello!
What is you doing?
Hello!

It's unclear why the second one will output "what is you doing?". Define the first time without setting true, the output is "hello". Do you have any ideas?

