I'm not sure what you are trying to achieve but your code (simplified) works just fine:
$a = array("asd", "qwe");
$asd = array("a" => 1, "b" => 2, "c" => 3);
$qwe = array("d" => 4, "e" => 5, "f" => 6);
foreach ($a as $item)
{
foreach ($$item as $key => $value)
{
echo $key . ": " . $value . "<br />";
}
}
Output:
a: 1
b: 2
c: 3
d: 4
e: 5
f: 6
Most likely one of your variables is empty (not an array) and that's why you receive that warning.