The spaceship operator <=> (introduced in PHP 7.0) is a three-way comparison operator that returns:

• -1 if left operand is less than right
• 0 if operands are equal
• 1 if left operand is greater than right

echo 1 <=> 2;  // -1
echo 2 <=> 2;  // 0
echo 3 <=> 2;  // 1

It's particularly useful for sorting callbacks and comparing multiple values.