I'm using format W-Y for weeknumber & year.
e.g. the final week of 2018 would be represented as '52-2018'.
But I can't get Carbon or DateTime to convert it back.
>>> Carbon::createFromFormat('W-Y', '01-2018')
InvalidArgumentException with message 'The format separator does not match
The separation symbol could not be found
Trailing data'
DateTime::createFromFormat (which is what Carbon extends) doesn't support the W formatting character, unfortunately.
The easiest way to work around this is to create a new DateTime (or Carbon) instance, and use the native setISODate method to set the year and week number:
$str = '01-2018';
list ($week, $year) = explode('-', $str);
$d = new DateTime;
$d->setISODate($year, $week);