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().