警告：为foreach（）提供的参数无效[重复]

I'm trying to make a shoping cart in which this product() is used for displaying what is in cart but in foreach statement it shows Warning: Invalid argument supplied for foreach()

function product()
{
  foreach(@$_SESSION as $name=>$value)
  {
    if($value>0)
    {
      if(substr($name,0,5) == 'cart_'){
        $id= substr($name,5,(strlen($name-5)));
        echo $id;
      }
    }
  }
}

</div>

$_SESSION is not set, it's null.

EVERY time you have a foreach nest it in an if:

if (isset($anything) && is_array($anything)) {
  foreach(@$anything ...) {
  }
}