大神们求解答

<?php

 $students = array(

'2010'=>array('令狐冲',"59"),

'2011'=>array('林平之',"44"),

'2012'=>array('曲洋',"89"),

'2013'=>array('任盈盈',"92"),

'2014'=>array('向问天',"93"),

'2015'=>array('任我行',"87"),

'2016'=>array('冲虚',"58"),

'2017'=>array('方正',"74"),

'2018'=>array('岳不群',"91"),

'2019'=>array('宁中则',"90"),

);//10个学生的学号、姓名、分数，用数组存储

foreach($students as $key=>$val)

{ //使用循环结构遍历数组,获取学号 

     echo $key; //输出学号

echo ":";

//循环输出姓名和分数

foreach($val as $v)

{

echo $v; 

}

echo "<br />";

}

?>

请问里面的$key $students $v $val都代表着什么？是根据什么定义的名字