php怎么实现转换为日期

$y=date("Y",time());         //年
$m=date("m",time());      //月
$d=date("d",time());        //日
echo $y."
";
echo $m."
";
echo $d."
";
$eight_clock = mktime(8, 0, 0, $m, $d ,$y);  //每天8点
echo date("Y-m-d H:i:s",$eight_clock)."
";
$day_time = mktime(0, 0, 0, $m, 1 ,$y);      //每月1号
echo date("Y-m-d H:i:s",$day_time)."
";

// 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], 
 );
}