array_walk() can't understand

Question

array(3) {
["a"]=>
string(8) "wuyanzhu"
["b"]=>
string(13) "huangxiaoming"
["c"]=>
string(8) "ninzetao"
}
a ---WUYANZHU
b ---HUANGXIAOMING
c ---NINZETAO
The effect after executing the custom function test_print: array(3) {
["a"]=>
string(8) "wuyanzhu"
["b"]=>
string(13) "huangxiaoming"
["c"]=>
string(8) "ninzetao"
}

strtoupper(); Convert string to uppercase; ——Why is the previous a not changed to uppercase?

array_walk(); I don’t understand this function. What do $item2 and $key mean and what do they do?

Answer 1

Read the manual. The manual is easy to understand, but you must first know the callback function

http://php.net/manual/zh/function.array-walk.php