$a = "hello";
$$a = " world";
/*
$a 的值为"hello"
$$a -> ${$a} ->$hello,也就是说$$a等价于$hello
*/
echo $a.$$a;//输出结果为 hello world

$v1 = 1;
$v2 = 2;
$v3 = 3;
$v4 = 4;
$v5 = 5;

$sum = 0;
for($i=1 ;$i<=5;$i++) {
  $v = "v".$i;
  $sum += $$v;
}
echo "\$sum = {$sum}<br />";

function testF1(){
  echo "可变函数的输出<br />";
}

$f1 = "testF1";
$f1();

//匿名函数，赋值给变量来使用
$anonymousfun = function (){
  echo "匿名函数的输出<br />";
};
$anonymousfun();

//匿名函数，作为函数参数使用
function anonymousDemo($a,$b,$f){
  $s1 = $a + $b;
  $s2 = $a - $b;
  $f($s1,$s2);
}

anonymousDemo(3,4,function ($m,$n){
  $x = $m * $n;
  echo "$m * $n = $x<br />";
})