So much criticism... As usual the php community is resisting any change. I think what you are doing has a lot of merit. Your DSL just needs some polish.

What I like:

- The new return "<-"

What I think should change:

- The concatenating assignment operator "%=" shouldn't change. It should stay the same ".="

- Accessing instance variables/methods should be by using '@' rather than '.' as that is becoming an implied standard and php folk are already used to using '.' for string concatenation.

- The setting of parameter types and defaults on methods is too heavy and not what people expect. I would put them back into method declaration.

  eg. 
  @str title=null
  class

  # To become
  class
  fn constructor(str @title = null)

  As @ would be the way to access instance variables what this means is the instance variable of title would be assigned the value that gets passed through the constructor.

  # This is too much magic imo  
  @arr row
    # List of integers.
    [!_->empty: "$row cannot be empty."]
  fn add_row(row)

  # it should be something like this:
  fn add_row(arr row)
    !"$row cannot be empty." if row->empty

    

- This doesn't make a lot of sense:

  for row in .rows
    html %= "<table>" | .render_row(row) | "</table>"

  # I would prefer something like:
  html .= @render_row(row) for row in @rows
  html = "<table>{html}</table>"

Good work on what you have already achieved, with a bit of extra polish I think Snow can become very desirable.