PHP - 如何保存简单的会话？

I am trying to save a value (0 or 1) in a session in PHP. I have my session_start() commands at the top but I cannot seem to get it working - it does return the saved number - it keeps returning 0 as in not saving the info - am I resetting something?

index.php

<?php

# session vars
session_start();
$session_id = session_id();

# set the session 
$sliderNumber=$_SESSION["CurrentSliderPosition"];

# check session
if($sliderNumber=="" || $sliderNumber=="0"){
    echo ' you are saved 0';
} else {
    echo ' saved your session is 1 - even though you have been on other pages';
}

<a href="make-session.php?slidePositionId=1">make 1 the session number</a><br />
<a href="make-session.php?slidePositionId=2">make 2 the session number</a><br />

?>

make-session.php

<?php
# get session number
$sliderNumber=$_REQUEST["slidePositionId"];

# session checker
if($sliderNumber=="0" || $sliderNumber==""){
    $_SESSION["CurrentSliderPosition"]=0;
} else {
    $_SESSION["CurrentSliderPosition"]=$sliderNumber;
}

 //echo $_SESSION["CurrentSliderPosition"];

 # redirect with our saved session
 header("Location: index.php");
?>

your make-session.php page doesn't have a session_start();, and it should.