Backtick operator does not parse interpolated variables

[Sjord]

The backtick execution operator can have variables that are interpolated, but these are not parsed.

<?php
`hello $world`;

Actual:

(program [0, 0] - [2, 0]
  (php_tag [0, 0] - [0, 5])
  (expression_statement [1, 0] - [1, 15]
    (shell_command_expression [1, 0] - [1, 14])))

Expected:

(program [0, 0] - [2, 0]
  (php_tag [0, 0] - [0, 5])
  (expression_statement [1, 0] - [1, 15]
    (shell_command_expression [1, 0] - [1, 14])))
      (string [1, 1] - [1, 7])
      (variable_name [1, 7] - [1, 13]
        (name [1, 8] - [1, 13])))))

[Sjord]

I created some tests for this.

[cfroystad]

Beautiful! Thanks for the tests. This should be fairly straight forward once all the quirks in heredoc has been ironed out