This loop is wonky (see the comment above referencing T13487) and the driving report for that task was some pathological nonsense, but probably-prefer this simpler loop body:

if (isset($text[$ii])) {
  $lines .= $text[$ii];
}

The advantage of this approach is that isset() is the same logical test (array key exists, and value is not null), but it's implemented as a PHP language keyword (isset) instead of a function, which is very very slightly faster than a function call.

In D20968, avoiding array_slice() + implode() gave us a 3x speedup on the highly secret pathological input, so it seems worth retaining this since the isset() code is a bit simpler anyway.