I like the HEREDOC syntax, e.g. for edge cases of generated HTML that are not worth putting into a template.
The only thing that annoys me about it, though, is that
Thank goodness this feature has finally landed in php 7.3 via RFC: Flexible Heredoc and Nowdoc Syntaxes
So now your example can cleanly be written as:
class myclass { function __construct() { $a = some_code(); $b = some_more_code(); $x = <<