When running Phing on PHP 7.1 I get a notice:
PHP Notice: A non well formed numeric value encountered in .composer/vendor/phing/phing/classes/phing/Phing.php on line 1636

The problem seems to be here: https://github.com/phingofficial/phing/blob/2.15.2/classes/phing/Phing.php#L1636

And seems to be related to this RFC: https://wiki.php.net/rfc/invalid_strings_in_arithmetic