Non capisco bene cosa intendi dire...
a quanto avevo capito $_SESSION è un array associativo, ovvero dentro ogni elemento di $_SESSION io posso metterci "quello che voglio" nel senso che posso fare:
Codice PHP:
$_SESSION['stringa'] = "stringa";
$_SESSION['numero'] = 3;
$_SESSION['oggetto'] = new ObjClass(); 
quindi, a quanto pensavo, credevo che avrei potuto richiamare i metodi dell'oggetto memorizzato in $_SESSION['oggetto'] semplicemente con:
Codice PHP:
$_SESSION['oggetto']->metodo1();
$_SESSION['oggetto']->metodo2(); 
dove metodo1() e metodo2() sono metodi della classe ObjClass.

Se non è possibile fare ciò, come posso allora memorizzare un oggetto per una sessione?

In caso fosse utile riporto anche il contenuto della classe VFSclient:
Codice PHP:
require('SOAP/Client.php');

/* METODI DEL WEBSERVICE
 * -- addResource(String TKEY, String url, String username, String pass)
 * -- synchronizeFolders(String first, String second, String TKEY)
 * -- upload(String encodedFile, String fileName, String To, String TKEY)
 * -- Rename(String Path, String oldName, String newName, String TKEY)
 * -- getAllResource(String TKEY)
 * -- getAllFile(String Resource, String TKEY)
 * -- login(String User, String Pass)
 * -- getInfo(String Resource, String TKEY)
 * -- logout(String TKEY)
 * -- addUser(String nick,String pass,String email)
 * -- deleteUser(String TKEY)
 * -- changeUserPwd(String TKEY, String newpass)
 * -- deleteResource(String TKEY, String url, String username, String pass)
 * -- changeResource(String TKEY, String url, String username, String pass, String new_url, String new_username, String new_pass)
 * -- copy(String From, String To, String TKEY)
 * -- delete(String Resource, String TKEY)
 * -- move(String From, String To, String TKEY)
 * -- download(String From, String TKEY)
 */

class VFSClient{
    
    private $WSDL_URL = 'VFService.wsdl';
    private $WSDL;
    private $client;
    public  $tkey;
    public  $err_msg;
    public  $res_arr = array();
    public  $myVSF   = array();
    
    public function __construct(){
        $this->WSDL    = new SOAP_WSDL($this->WSDL_URL); 
        $this->client  = $this->WSDL->getProxy();
    }
    
    public function getErrorMessage(){
        return $this->err_msg;
    }
    
    public function getTKEY(){
        return $this->tkey;
    }
    
    public function login($username, $password){
        $ret = $this->client->login($username,$password);
        $arr = explode("|", $ret);
        if ($arr[0]==1) {
            // LOGIN SUCCESSFUL
            $this->tkey = $arr[1];
            return true;
        } else {
            // LOGIN FAILED
            $this->err_msg = $arr[1];
            return false;
        }
    }
    
    public function logout(){
        $ret = $this->client->logout($this->tkey);
        $arr = explode("|", $ret);
        if ($arr[0]==1) {
            // LOGOUT SUCCESSFUL
            return true;
        } else {
            // LOGOUT FAILED
            $this->err_msg = $arr[1];
            return false;
        }
    }
    
    public function userAdd($username, $password, $email){
        $ret = $this->client->addUser($username, $password, $email);
        $arr = explode("|", $ret);
        if ($arr[0]==1) {
            // USERADD SUCCESSFUL
            return true;
        } else {
            // USERADD FAILED - USER NOT ADDED IN DATABASE
            $this->err_msg = $arr[1];
            return false;
        }
    }
    
    public function userDel(){
        $ret = $this->client->deleteUser($this->tkey);
        $arr = explode("|", $ret);
        if ($arr[0]==1) {
            // USERDEL SUCCESSFUL
            return true;
        } else {
            // USERDEL FAILED - USER IS STILL IN DATABASE
            $this->err_msg = $arr[1];
            return false;
        }
    }
    
    public function userMod($newpassword){
        $ret = $this->client->changeUsrPwd($this->tkey, $newpassword);
        $arr = explode("|", $ret);
        if ($arr[0]==1) {
            // SUCCESS - PASSWORD CHANGED TO NEWPASSWORD
            return true;
        } else {
            // FAILED - PASSWORD HAS NOT BEEN CHANGED
            $this->err_msg = $arr[1];
            return false;
        }
    }
    
    public function resAdd($url, $username, $password){
        $ret = $this->client->addResource($this->tkey, $url, $username, $password);
        $arr = explode("|", $ret);
        if ($arr[0]==1) {
            // SUCCESS - NEW RESOURCE ADDED IN DATABASE
            return true;
        } else {
            // FAILED - RESOURCE HAS NOT BEEN ADDED
            $this->err_msg = $arr[1];
            return false;
        }
    }
    
    public function resDel($url, $username, $password){
        $ret = $this->client->changeResource($this->tkey, $url, $username, $password);
        $arr = explode("|", $ret);
        if ($arr[0]==1) {
            // SUCCESS - RESOURCE DELETED
            return true;
        } else {
            // FAILED - RESOURCE HAS NOT BEEN DELETED
            $this->err_msg = $arr[1];
            return false;
        }
    }
    
    public  function resMod($old_url, $old_user, $old_password, $new_url, $new_user, $new_password){
        $ret = $this->client->changeResource($this->tkey, $old_url, $old_user, $old_password, $new_url, $new_user, $new_password);
        $arr = explode("|", $ret);
        if ($arr[0]==1) {
            // SUCCESS - RESOURCE HAS BEEN CHANGED
            return true;
        } else {
            // FAILED - RESOURCE HAS NOT BEEN CHANGED
            $this->err_msg = $arr[1];
            return false;
        }
    }
    
    public function getRes(){
        $ret = $this->client->getAllResource($this->tkey);
        $arr = explode("|", $ret);
        if ($arr[0]==1) {
            // SUCCESS
            for($i=1; i<count($arr); $i++){
                // MEMORIZZO IL VETTORE DELLE RISORSE DISPONIBILI
                $res[$i-1] = $arr[$i];
            }
            return $res;
        } else {
            // FAILED
            $this->err_msg = $arr[1];
            return false;
        }
    }
    
    public function getFiles($res){
        $ret = $this->client->getAllFiles($res, $this->tkey);
        $arr = explode("|", $ret);
        if ($arr[0]==1) {
            // SUCCESS
            for($i=1; i<count($arr); $i++){
                /* Struttura del vettore $files:
                 *  - $files[nomeFile][tipoFile]
                 * Scorrere il vettore:
                 *   foreach(array_keys($files) as $key){
                 *       $nomeFile = $key;
                 *       $tipoFile = $files[$key]
                 *   }
                 */
                $files[substr($arr[$i],2)] = $arr[$i]{0};
            }
            return $files;
        } else {
            // FAILED
            $this->err_msg = $arr[1];
            return false;
        }
    }
    
    public function getFileInfo($file){
        $ret = $this->client->getInfo($file, $this->tkey);
        $arr = explode("|", $ret);
        if ($arr[0]==1) {
            // SUCCESS
            for($i=1; i<count($arr); $i++){
                $tmp = explode("$", $arr[$i]);
                /* Struttura del vettore $fileInfo:
                 *  - $fileInfo[tipoInfo][valoreInfo]
                 * Scorrere il vettore:
                 *   foreach(array_keys($fileInfo) as $key){
                 *       $tipoInfo = $key;
                 *       $valoreInfo = $fileInfo[$key]
                 *   }
                 */
                $fileInfo[$tmp[0]] = $tmp[1];
            }
            return $fileInfo;
        } else {
            // FAILED
            $this->err_msg = $arr[1];
            return false;
        }
    }
    
    public function copy($src, $dst){
        $ret = $this->client->copy($src, $dst, $this->tkey);
        $arr = explode("|", $ret);
        if ($arr[0]==1) {
            // SUCCESS - COPY DONE
            return true;
        } else {
            // FAILED - COPY FAILED
            $this->err_msg = $arr[1];
            return false;
        }
    }
    
    public function move($src, $dst){
        $ret = $this->client->move($src, $dst, $this->tkey);
        $arr = explode("|", $ret);
        if ($arr[0]==1) {
            // SUCCESS - MOVE DONE
            return true;
        } else {
            // FAILED - MOVE FAILED
            $this->err_msg = $arr[1];
            return false;
        }
    }
    
    public function remove($file){
        $ret = $this->client->delete($file, $this->tkey);
        $arr = explode("|", $ret);
        if ($arr[0]==1) {
            // SUCCESS - REMOVE DONE
            return true;
        } else {
            // FAILED - REMOVE FAILED
            $this->err_msg = $arr[1];
            return false;
        }
    }
    
    public function rename($path, $oldfilename, $newfilename){
        $ret = $this->client->rename($path, $oldfilename, $newfilename, $this->tkey);
        $arr = explode("|", $ret);
        if ($arr[0]==1) {
            // SUCCESS - RENAME DONE
            return true;
        } else {
            // FAILED - RENAME FAILED
            $this->err_msg = $arr[1];
            return false;
        }
    }
    
    // Si deve specificare come secondo parametro il path di destinazione del file scaricato
    public function download($src, $dst){
        $ret = $this->client->download($src, $this->tkey);
        $arr = explode("|", $ret);
        if ($arr[0]==1) {
            // SUCCESS
            /* La funzione download ritorna il nome del file 
             * ed il file codificato base64 sottoforma di stringa.
             */
            $filename = $arr[1];
            $data     = base64_decode($arr[2]);
            $handle   = fopen($dst.'/'.$filename, 'w');
            if($handle != false){
                if(fputs($handle, $data, strlen($data)) != false){
                    return true;
                } else {
                    // fputs FAILED
                    return false;
                }
            } else {
                // fopen FAILED
                return false;
            }
        } else {
            // FAILED
            $this->err_msg = $arr[1];
            return false;
        }
    }
    
    // $file Ã¨ il path completo del file da uploadare
    public function upload($file, $res){
        $handle = fopen($file, 'r');
        if($handle != false){
            $data = fread($handle, filesize($file));
            if($data != false){
                $base64 = base64_encode($data);
                $ret = $this->client->upload($base64, basename($file), $res, $this->tkey);
                $arr = explode("|", $ret);
                if($arr[0]==1){
                    //SUCCESS - FILE UPLOADED
                    return true;
                } else {
                    //FAILED
                    $this->err_msg = $arr[1];
                    return false;
                }
            } else {
                // fread FAILED
                return false;
            }
        } else {
            //fopen FAILED
            return false;
        }
    }
    
    public function synchronize($dir1, $dir2){
        $ret = $this->client->synchronizeFolders($dir1, $dir2, $this->tkey);
        $arr = explode("|", $ret);
        if($arr[0]==1){
            //SUCCESS - FILE UPLOADED
            return true;
        } else {
            //FAILED
            $this->err_msg = $arr[1];
            return false;
        }
    }
    
} 
Dal messaggio di errore:
Please ensure that the class definition "WebService_VFSiService_VFService" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in C:\Programmi\Apache Software Foundation\Apache2.2\htdocs\VFSclient\VFSclient.cl ass.php on line 49
mi sembra di capire che l'errore non stia nel richiamare il metodo login della classe VFSclient(), ma piuttosto si verifichi quando viene eseguita l'istruzione:
Codice PHP:
$ret = $this->client->login($username,$password); 
che si trova all'interno del metodo login della classe VFSclient.

Non riesco però a capire cosa fare per risolvere questa situazione...