1. 准备一个标准crontab文件 ./crontab
复制代码 代码如下:
# m h dom mon dow command
* * * * * date > /tmp/cron.date.run

2. crontab -e 将此cron.php脚本加入系统cron

复制代码 代码如下:
* * * * * /usr/bin/php cron.php

3. cron.php 源码

复制代码 代码如下:
// 从./crontab读取cron项，也可以从其他持久存储（mysql、redis）读取
$crontab = file('./crontab');
$now = $_SERVER['REQUEST_TIME'];

foreach ( $crontab as $cron ) {
 $slices = preg_split("/[\s]+/", $cron, 6);
 if( count($slices) !== 6 ) continue;

 $cmd       = array_pop($slices);
 $cron_time = implode(' ', $slices);
 $next_time = Crontab::parse($cron_time, $now);
 if ( $next_time !== $now ) continue; 

 $pid = pcntl_fork();
 if ($pid == -1) {
  die('could not fork');
 } else if ($pid) {
  // we are the parent
  pcntl_wait($status, WNOHANG); //Protect against Zombie children
 } else {
      // we are the child
  `$cmd`;
  exit;
 }
}

/* https://github.com/jkonieczny/PHP-Crontab */
class Crontab {
   /**
 * Finds next execution time(stamp) parsin crontab syntax,
 * after given starting timestamp (or current time if ommited)
 *
 * @param string $_cron_string:
 *
 * 0 1 2 3 4
 * * * * * *
 * - - - - -
 * | | | | |
 * | | | | +----- day of week (0 - 6) (Sunday=0)
 * | | | +------- month (1 - 12)
 * | | +--------- day of month (1 - 31)
 * | +----------- hour (0 - 23)
 * +------------- min (0 - 59)
 * @param int $_after_timestamp timestamp [default=current timestamp]
 * @return int unix timestamp - next execution time will be greater
 * than given timestamp (defaults to the current timestamp)
 * @throws InvalidArgumentException
 */
    public static function parse($_cron_string,$_after_timestamp=null)
    {
        if(!preg_match('/^((\*(\/[0-9]+)"Invalid cron string: ".$_cron_string);
        }
        if($_after_timestamp && !is_numeric($_after_timestamp)){
            throw new InvalidArgumentException("\$_after_timestamp must be a valid unix timestamp ($_after_timestamp given)");
        }
        $cron = preg_split("/[\s]+/i",trim($_cron_string));
        $start = empty($_after_timestamp)?time():$_after_timestamp;

        $date = array( 'minutes' =>self::_parseCronNumbers($cron[0],0,59),
                            'hours' =>self::_parseCronNumbers($cron[1],0,23),
                            'dom' =>self::_parseCronNumbers($cron[2],1,31),
                            'month' =>self::_parseCronNumbers($cron[3],1,12),
                            'dow' =>self::_parseCronNumbers($cron[4],0,6),
                        );
        // limited to time()+366 - no need to check more than 1year ahead
        for($i=0;$i<=60*60*24*366;$i+=60){
            if( in_array(intval(date('j',$start+$i)),$date['dom']) &&
                in_array(intval(date('n',$start+$i)),$date['month']) &&
                in_array(intval(date('w',$start+$i)),$date['dow']) &&
                in_array(intval(date('G',$start+$i)),$date['hours']) &&
                in_array(intval(date('i',$start+$i)),$date['minutes'])

                ){
                    return $start+$i;
            }
        }
        return null;
    }

    /**
 * get a single cron style notation and parse it into numeric value
 *
 * @param string $s cron string element
 * @param int $min minimum possible value
 * @param int $max maximum possible value
 * @return int parsed number
 */
    protected static function _parseCronNumbers($s,$min,$max)
    {
        $result = array();

        $v = explode(',',$s);
        foreach($v as $vv){
            $vvv = explode('/',$vv);
            $step = empty($vvv[1])?1:$vvv[1];
            $vvvv = explode('-',$vvv[0]);
            $_min = count($vvvv)==2?$vvvv[0]:($vvv[0]=='*'?$min:$vvv[0]);
            $_max = count($vvvv)==2?$vvvv[1]:($vvv[0]=='*'?$max:$vvv[0]);

            for($i=$_min;$i<=$_max;$i+=$step){
                $result[$i]=intval($i);
            }
        }
        ksort($result);
        return $result;
    }
}