in PHP7? and? ? What's the difference

This article will introduce to you the difference between "?" and "??" in PHP7. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to everyone.

$a = ''; // or 0 or false

$b = $a ?? 'a';
// 此时会判断$a是否存在 $a不为null 
// 等价于
// $b = isset($a) ? $a : 'a';
// $b is '' or 0 or false

$c = $a ?: 'a';
// 此时会判断$a的值
// $c is 'a'

$a = null;

$b = $a ?? 'a';
// 此时$a为null
// $b is  'a'

$c = $a ?: 'a';
// $c is 'a'

$a = null;
$b = 'b';
$c = $a ?? $b ?? 'c';
// 返回第一个有定义的值
// $c is 'b'

$a = null;
$b = null;
$c = $a ?? $b ?? 'c';
// $c is 'c'

function getId(?int $id) {
    return $id;
}
// 参数为指定的整型或空值
getId(857); // 857
getId('857'); // 参数非整型 报错
getId(); // 参数空 报错
getId(''); // 参数非整型 报错
getId(0); // 0
getId(null); // null

function getId():?int
{
	return 1;
}
// 返回值为指定的整型或空值
getId() // 1

function getId():?int
{
	return null;
}
getId() // null

function getId():?int
{
	return '1';
}
getId() // 返回非整型 报错

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