PHP $ _POST字符串到JSON数组

Question

I'm trying to decode json string using following code

$jsonString = "[\"[e-mail]\",\"[e-mail]\",\"[e-mail]\"]";
$arrayOfEmails=json_decode($jsonString);

This code works fine. But When I $_POST this string via form & then try to decode on other page, It shows NULL.

I checked for string value after post on other page & it returns as "[\"[e-mail]\",\"[e-mail]\",\"[e-mail]\"]";

But when I try decode this $_POST string as follow, It shows NULL.

if ($_POST) {
    $string = $_POST['my_string'];

    $arrayOfEmails=json_decode($string);
}

What I'm doing wrong ?