PHP单位注意：数组到字符串转换

I'm using strtotime() in PHP to get the days of a week (I made a calendar, with different views, so it changes). PHP Unit gives me the warning of Array to string conversion whenever I try this:

$monday  = date('Y-m-d', strtotime($sunday . '+ 1 day'));

$sunday is a variable that I get this way:

public function GetWeekRange($date) {
        $ts = strtotime($date);
        $start = (date('w', $ts) == 0) ? $ts : strtotime('last sunday', $ts);
        return date('Y-m-d', $start);
}

and $date is a string in the format 'YYYY-mm-dd'

I'm not sure why I get this error, the code works just fine, but the test gives that warning, and it's going to give a warning on my environment where all the different projects are shown (red/green depending on whether there's a failure or not).

Thanks,

Fer

Rookie mistake: I updated the function to return only one string, but the mocked value I kept as an array.

My bad.