Encapsulating PHP Session

One of my colleagues moved to a different company and I got his source code of an existing project. I was shocked how messy the code looked. I am not a PHP developer so maybe this is a stupid question but it seems to me that he uses the $_SESSION too much and we get many errors regarding the Sessions. I was wondering if it would be good to encapsulate the $_SESSION and write for each value he uses an own method.

The problem I see with this code is that he uses the session object like this

$_SESSION['customer']['items'] = getItems() //returns an array
$_SESSION['article'][$Id]['name'] = utf8_decode[$received[1]];

So from my point of view I would store all his stuff in simple Popos and place those into the Session.

So the Popo for the customer would look like this

class CustomerPopo
{

    private $_id;

    private $_salutation;

    private $_name;

    private $_surename;

        public function getId()
    {
        return $this->_id;
    }

    public function setId($value)
    {
        $this->_id = $value;
    }

    public function getSalutation()
    {
        return $this->_salutation;
    }

    public function setSalutation($value)
    {
        $this->_salutation = $value;
    }

    public function getName()
    {
        return $this->_name;
    }

    public function setName($value)
    {
        $this->_name = $value;
    }

    public function getSurename()
    {
        return $this->_surename;
    }

    public function setSurename($value)
    {
        $this->_surename = $value;
    }

    function CustomerPopo() {

    }
}

And I imagined the SessionManager like this

class SessionManager
{

    private static function getValue($valueName)
    {
        $value = SessionManager::getValueFromSession($valueName);
        if (is_null($value)) {
             //Handle stuff and do further checks
        }
        return $value;
    }

    private static function getValueFromSession($valueName)
    {
        $value = null;
        if (isset($_SESSION[$valueName])) {
            $value = $_SESSION[$valueName];
        }
        return $value;
    }

    private static function setValue($valueName, $value)
    {
        $_SESSION[$valueName] = $value;
    }

    private static function clearValue($valueName)
    {
        if (isset($_SESSION[$valueName])) {
            unset($_SESSION[$valueName]);
        }
    }

    public static function getCustomer()
    {
        $customer = '';
        try {
            $customer = SessionManager::getValue('customer');
        } catch (Exception $e) {
            $customer = '';
        }
        return $customer;
    }

    public static function setCustomer($customer)
    {
        SessionManager::setValue('customer', $customer);
    }
}

With this I could kill some of the errors which arise from different spellings of the word customer/Customer/cusomter. I imagine the SessionManager would grow big because we have ~30 Session variables in the code.

Is there a design pattern which I could follow to implement such a SessionManager without making it worse in the end ? as stated I am not very familiar with PHP (yet).

EDIT:

Since the approach seems to be valid how can I handle the 30 Session variables i mentioned ? If I have a set/get and maybe a clear method for each value i will end up with 60-90 methods