php get day of week

Things to be aware of when using week numbers with years.

<?php
echo date("YW", strtotime("2011-01-07")); // gives 201101
echo date("YW", strtotime("2011-12-31")); // gives 201152
echo date("YW", strtotime("2011-01-01")); // gives 201152 too
?>

BUT

<?php
echo date("oW", strtotime("2011-01-07")); // gives 201101
echo date("oW", strtotime("2011-12-31")); // gives 201152
echo date("oW", strtotime("2011-01-01")); // gives 201052 (Year is different than previous example)
?>

Reason:
Y is year from the date
o is ISO-8601 year number
W is ISO-8601 week number of year

Conclusion:
if using 'W' for the week number use 'o' for the year.

$dateTime = new \DateTime();
/**
 * You can get the string by using format
 */
$dateTime->format('Y-m-d H:i:s');

$dayofweek = date('w', strtotime($date));
$result    = date('Y-m-d', strtotime(($day - $dayofweek).' day', strtotime($date)));

date('w'); //gets day of week as number(0=sunday,1=monday...,6=sat)

//note:returns 0 through 6 but as string so to check if monday do this:
if(date('w') == 1){
	echo "its monday baby";
}