Whenever you need to det day like Monday, Tuesday etc from full date. i mean you have any specific date like "2015-10-10" and you want to get day then you can do in both way first using strtotime() and second using DateTime object.
In Following example you can see i give you both way to get day from your date. now i added format 'l' so it will return full name of day like "Tuesday", but if you want to get short name of day then you can change format like "D".
// Using strtotime()
$specificDate = strtotime('2016-2-3');
$specificDate = date('l', $specificDate);
// Using DateTime Object
$yourDate = "2011-12-2";
$yourDate = DateTime::createFromFormat("Y-m-d", $yourDate);