PHP short-ternary ("Elvis") operator vs null coalescing operator

Can someone explain the differences between [ternary operator shorthand](https://www.php.net/manual/en/language.operators.comparison.php#language.operators.comparison.ternary) (`?:`) and [null coalescing operator](https://www.php.net/manual/en/language.operators.comparison.php#language.operators.comparison.coalesce) (`??`) in PHP? When do they behave differently and when in the same way (if that even happens)? $a ?: $b VS. $a ?? $b