&引用

$b =1;
$a=&$b;
$a=100;
echo $a;//100
echo $b;//100

$c=1;
$d=&$c;
$d=200;
echo $c;//200
echo $d;//200

//引用就是使用同一个内存地址（引用之间的变量相当于别名）
$array = [1, 2, 3];
foreach ($array as $key => &$val) {
    //$val=array[$key];
    $val+=1;//相当于array[$key]+=1;
    print_r($array);
}
 print_r($array);//[2,3,4]



$arr = [1,2,4];

foreach($arr as &$a){
    $a = $a*$a;
    print_r($a);//1-4-16
}

print_r($arr);//[1,4,16]

foreach($arr as $a){
    print_r($a);//1-4-4
}