Comparing two values is a frequent operation in any programming language. We use various language operators to express the type of comparison we wish to execute between two variables. In PHP, these operators include equal ($a == $b), identical ($a === $b), not equal ($a != $b or $a <> $b), not identical ($a !== $b), less than ($a < $b), greater than ($a > $b), less than or equal to ($a <= $b), and greater than or equal to ($a >= $b) comparisons.

All of these comparison operators result in Boolean true or false. Sometimes, however, there are cases where a three-way comparison is needed, in which case, the result of the comparison is more than just a Boolean true or false. While we can achieve a three-way comparison using various operators through various expressions, the solution is all but elegant.

With the release of PHP 7, a new spaceship <=> operator has been introduced, with a syntax as follows:

(expr) <=> (expr)

The spaceship <=> operator offers combined comparison. After comparison, it follows these conditions:

Comparison rules used to yield the preceding results are the same as those used by existing comparison operators: <, <=, ==, >=, and >.

The usefulness of the new operator is especially apparent with ordering functions. Without it, the ordering functions were quite robust, as per the following example:

$users = ['branko', 'ivana', 'luka', 'ivano'];

usort($users, function ($a, $b) {
  return ($a < $b) ? -1 : (($a > $b) ? 1 : 0);
});

We can shorten the preceding example by applying the new operator to it, as follows:

$users = ['branko', 'ivana', 'luka', 'ivano'];

usort($users, function ($a, $b) {
  return $a <=> $b;
});

Applying the spaceship <=> operator, where applicable, gives the expressions simplicity and elegance.