未定义的变量：_SESSION [CORE / src / Network / Session.php CakePHP 3.0

I am using CakePHP 3.0, and uploaded it to the server, it was working fine in local but somehow after uploading it to server it shows error

Notice (8): Undefined variable: _SESSION [CORE/src/Network/Session.php, line 438]

If any have idea about that please help me. I am still trying to solve it.
I have also tried to put session_start(); in AppController and many other places but then it goes blank.

I have found the issue, Its when I was uploading the files to servers it was automatically appending the white blank lines at the end of the AppController and also another controller files.

So when I removed the blank lines from that files it is working fine again

    //To detect the file which have issue or extra space or early header start
//File location : vendor/cakephp/cakephp/src/Network/Session.php 
    public function write($name, $value = null)
            {
                if (empty($name)) {
                    return;
                }

                if (!$this->started()) {
                    $this->start();
                }

                $write = $name;
                if (!is_array($name)) {
                    $write = [$name => $value];
                }

                //remove the comment from bottom line and run your code it will 
                //show you the exact file from where you have to remove the blank space
                //session_start();
                $data = $_SESSION ?: [];
                foreach ($write as $key => $val) {
                    $data = Hash::insert($data, $key, $val);
                }

                $this->_overwrite($_SESSION, $data);
            } 

Small issue but took long time to detect it,hope this will help you all.

Place this in your php.ini file on your server

session.save_path = “/var/tmp”

You need make sure to start the session at the top of every PHP file where you want to use the $_SESSION superglobal. Like this:

<?php
  session_start();
  echo $_SESSION['youritem'];
?>

You forgot the Session HELPER.

Check this link : book.cakephp.org/2.0/en/core-libraries/helpers/session.html