I have this at the very top of my send.php file:
ob_start();
@session_start();
//some display stuff
$_SESSION[\'id\'] = $id; //$id has a value
If you use a configured vhost and find the same error then you can override the default setting of php_value session.save_path under your
#
# Apache specific PHP configuration options
# those can be override in each configured vhost
#
php_value session.save_handler "files"
php_value session.save_path "/var/lib/php/5.6/session"
php_value soap.wsdl_cache_dir "/var/lib/php/5.6/wsdlcache"
Change the path to your own '/tmp' with chmod 777.