如何把日期转换成前n天？

// convert a date into a string that tells how long ago

// that date was.... eg: 2 days ago, 3 minutes ago.

function ago($d) {

 $c = getdate();

 $p = array('year', 'mon', 'mday', 'hours', 'minutes', 'seconds');

 $display = array('year', 'month', 'day', 'hour', 'minute', 'second');

 $factor = array(0, 12, 30, 24, 60, 60);

 $d = datetoarr($d);

 for ($w = 0; $w < 6; $w++) {

 if ($w > 0) {

  $c[$p[$w]] += $c[$p[$w-1]] * $factor[$w];

  $d[$p[$w]] += $d[$p[$w-1]] * $factor[$w];

 }

 if ($c[$p[$w]] - $d[$p[$w]] > 1) { 

  return ($c[$p[$w]] - $d[$p[$w]]).' '.$display[$w].'s ago';

 }

 }

 return '';

}







// you can replace this if need be. 

// This converts my dates returned from a mysql date string 

// into an array object similar to that returned by getdate().

function datetoarr($d) {

 preg_match("/([0-9]{4})(\\-)([0-9]{2})(\\-)([0-9]{2})([0-9]{2})(\\:)([0-9]{2})(\\:)([0-9]{2})/",$d,$matches);

 return array( 

 'seconds' => $matches[10], 

 'minutes' => $matches[8], 

 'hours' => $matches[6], 

 'mday' => $matches[5], 

 'mon' => $matches[3], 

 'year' => $matches[1], 

 );

}