when post a var contains "." then the "." changed to "_"

Question

for example,in page1 I sended :

test.info ：1;

then in page2:

echo $_REQUEST['test.info'];
echo $_REQUEST['test_info'];

first one is null ,second is 1 it changed to test_info

I just don't know the reason why the $key changed .

Answer 1

It's from the old days of php, when post, get etc. variables were turned into regular global variables. But as these have the limitation that they cannot contain dots, dots were turned into _

$test.info - invalid variable name

$test_info - valid