// ??

$a = null;

$b = $a ?? "c";

var_dump($b); //string(1) "c"

// 太空船操作符

$a = 1;

$b = 2;

var_dump($a <=> $b); //int(-1)

$a = 1;

$b = 1;

var_dump($a <=> $b); //int(0)

$a = 2;

$b = 1;

var_dump($a <=> $b); //int(1)