在wordpress中重新加载页面后销毁的php会话变量值

I have a problem with sessions in wordpress. First I've activated the use of sessions in my functions.php and no I get the right variable but just after I reload my target site after setting the variable. When I load another page first and my target page seacondly my variable has a wrong value without any context to my first value. When I have the right value on my target site and I reload this site the value is also wrong. This is my code:

//Activate sessions() in functions.php
add_action('init', function(){
      if(!session_id())
      {
          session_start();
      }
}, 1);

//Calculate the variable
$gesamtumsatz_kcal = round (($value1 + $value2 + $value3), 0); 

//Then set the session variable
$_SESSION['gesamtumsatz_kcal'] = $gesamtumsatz_kcal;

//Get my session variable on another page somewhere on my site
if(isset($_SESSION['gesamtumsatz_kcal'])) {
    $gesamtumsatz_kcal = $_SESSION['gesamtumsatz_kcal'];
} else {
    $gesamtumsatz_kcal = '';
}

//Echo my variable
echo $gesamtumsatz_kcal;

Do you have any idea whats wrong? I'm absolutely at the end with this....

Thanks a lot!

• • • • • • • • I have to add this

When I define a constant variable without calculate something like this:

//Define the variable
$_SESSION['test'] = 1056;

//Get the variable
$test = $_SESSION['test'];
    
//Echo the variable
echo $test;

I always get the right value: 1056 back... What is this? Can't uderstand

</div>

wordpres destroys session when it is not opened. so use

if(!session_id()) session_start();

in the start of wp-config.php or better in the init of your code/plugin