引用变量定义：用不同的名字访问同一变量内容以下两段代码$b与$a均指向同一内存空间Tips: 使用 unset() 只会取消引用，不会销毁内存空间$a = 1;$b = $a;$a = 1;$b = &$a;Example$data = [‘a’, ‘b’, ‘c’];foreach ($data as $key => $val) { $val = &$data[$key]; print_r($data);}/loop_1$key = 0$val = ‘a’$val = &$data[0] =>‘a’ (此时$val与$data[0]指向同一地址)$data = [‘a’,‘b’,‘c’];loop_2$key = 1$val = ‘b’ => $data[0] = ‘b’ ($data[0]与$val地址相同，$val改变，$data[0]的值也发生改变)$val = &$data[1] => $val = ‘b’（$val地址再次发生改变，与$data[1]相同）$data = [‘b’,‘b’,‘c’];loop_3$key = 2$val = ‘c’ => $data[1] = ‘c’($data[1]与$val地址相同，$val改变，$data[1]的值也发生改变)$val = &$data[2] => $val =>‘c’（$val地址再次发生改变，与$data[2]相同）$data = [‘b’,‘c’,‘c’];/