This constant gives you the name of the function it’s used in. The case is preserved.

If you’re trying to think about when this could be useful and can’t figure it out, I understand. After all, the function’s name isn’t going to change, right? (It could before you’re done)

function monKey() {
  return "I'm in: " . __FUNCTION__;
}
echo monKey(); // I'm in: monKey

It also works as the default value of a parameter.

function monKey($text = __FUNCTION__) {
  return "I'm in: " . $text;
}
echo monKey(); // I'm in: monKey

You can’t fool this magic constant. Even if a function calls a function, you get the name of the function it is actually used in.

function monKey() {
  return butLer();
}
 
function butLer() {
  return "I'm in: " . __FUNCTION__;
}
 
echo monKey(); // I'm in: butLer