Try this:
date_default_timezone_set('Asia/Kolkata');
$date = '2017/06/07';
$date = str_replace('/','-', $date);
$weekday = date('l', strtotime($date));
echo $weekday;
// Output: Wednesday
Phpfiddle link
Check the document here
Note: Dates in the m/d/y or d-m-y formats are disambiguated by looking
at the separator between the various components: if the separator is a
slash (/), then the American m/d/y is assumed; whereas if the
separator is a dash (-) or a dot (.), then the European d-m-y format
is assumed. If, however, the year is given in a two digit format and
the separator is a dash (-, the date string is parsed as y-m-d. To
avoid potential ambiguity, it's best to use ISO 8601 (YYYY-MM-DD)
dates or DateTime::createFromFormat() when possible.