Weird PHP error: 'Can't use function return value in write context'

This also happens when using empty on a function return:
!empty(trim($someText)) and doSomething()

because empty is not a function but a language construct (not sure), and it only takes variables:
Right:
empty($someVar)

Wrong:
empty(someFunc())

Since PHP 5.5, it supports more than variables. But if you need it before 5.5, use trim($name) == false. From empty documentation.

You mean
if (isset($_POST[‘sms_code’]) == TRUE ) {

though incidentally you really mean
if (isset($_POST[‘sms_code’])) {

Weird PHP error: ‘Can’t use function return value in write context’