PHP 三元运算 ??与?:

//$c = $a ?? $b;  等效  $c = isset($a) ? $a : $b;

//$c = $a ?: $b; 等效  $c = $a ? $a : $b;

$a = 'a';

$b = 'b';

$c = $a ?? $b; //a

$c = $a ?: $b; //a

$a = false;

$b = 'b';

$c = $a ?? $b; //false

$c = $a ?: $b; //b