First of all you should execute this code :
<?php
session_start();
$_SESSION["A"] = "Some Value";
?>
then you should execute this one :
<?php
start_session();
$_SESSION["A"] = "Some New Value"; // set new value
session_reset(); // old session value restored
echo $_SESSION["A"];
//Output: Some Value
?>
That is because session_reset() is rolling back changes to the last saved session data, which is their values right after the session_start().