I had to update line 43 of WA_eCart_Definition_PHP.php from:
function __construct(
to:
function WA_eCartDefinition(
Which is an older style syntax that should essentially mean the same thing. Maybe this is an issue with the specific version of PHP you are running. This should fix the problem, but if you update your cart object it may overwrite my update. Please note this fix and/or save a backup of the file in case you run into this issue again.