Subdomain filter - Only variables should be passed by reference

because array_shift expects a pointer to an array explode is giving an array, but because your not saving it to a variable your not actually passing a pointer to it you passing the whole array.

$array = explode('.', $_SERVER['HTTP_HOST']);
$subdomain = array_shift($array);

A bit more of an explanation.

explode('.', $_SERVER['HTTP_HOST']) Creates an array, $array = means set $array to be a pointer/refference to that array

array_shift take a pointer/reference to an array it will not accept a array directly, this used to be denoted by array_shift(&$array);

but the reason for this is because array_shift alters the original array so if you don't have access to the original array saved it's going to do work that you then can't access so it's pointless load