Add days to date in PHP

La siguiente función sumará un número determinado de días a un fecha dada.

	/**
	* Add days to a date.
	*
	* @param string $d, date
	* @param int $ndays, number of days
	* @param string $format, format date (@see: http://php.net/manual/en/function.date.php)
	*/
	function add_days($ndays, $d, $format="Y-m-d") {
	$p = $ndays > 1 ? "s" : "";
	$nf = strtotime(date($format, strtotime($d)) . " +".$ndays." day". $p);
	return date($format, $nf);
	}

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Referencias:
http://www.vision.to/how-to-add-days-weeks-months-to-any-date-.php
http://php.net/manual/en/function.date.php
http://php.net/manual/en/function.strtotime.php