PHP7 ?:和??的区别

$z = $x ?? $y;
//等价于下面
$z = isset($x) ? $x : $y;

$z = $x ?: $y;
//等价于下面
$z = $x ? $x : $y;

实例1：

<?php  
$x = 1; $y = 2;
$z = $x ?? $y;
echo 'z:'.$z;
$zz = $x ?: $y;
echo 'zz:'.$zz;
//输出是
z:1zz:1

实例2：

$x = 0; $y = 2;
$z = $x ?? $y;
echo 'z:'.$z;
$zz = $x ?: $y;
echo 'zz:'.$zz;
//输出是
z:0zz:2

实例3：

 $y = 2;
$z = $x ?? $y;
echo 'z:'.$z;
$zz = $x ?: $y;
echo 'zz:'.$zz;
//输出是
z:2
Notice: Undefined variable: x in D:PHPWAMP_IN2phpwampserverNginx-PHPWNMPhtdocs	est2.php on line 5
zz:2