PHP, crontab, DateInterval: Convert human-readable interval to crontab format

I'm having a little confusion with crontab interval format. The point is that i want to get intervals from human-readable strings like "20 minutes", "16 hours and 30 minutes". This is done by PHP DateTime already. But what I need is passing crontab-valid string as exec(sprintf('echo "%s %s %s * * %s" | crontab', $minute, $hour, $day, $command));. Anyhow, here is sample PHP script

<?php
function getCrontabInterval($timestring)
{
    $interval = DateInterval::createFromDateString($timestring);

    $minute = $interval->i > 0 ? "*/{$interval->i}" : '*';
    $hour = $interval->h > 0 ? "*/{$interval->h}" : '*';
    $day = $interval->d > 0 ? "*/{$interval->d}" : '*';
    $crontab = sprintf('echo "%s %s %s * * %s" | crontab', $minute, $hour, $day, '%command%');

    echo "Days:\t\t", $interval->d, "\n",
        "Hours:\t\t", $interval->h, "\n",
       开发者_如何学运维 "Minutes:\t", $interval->i, "\n",
        "COMMAND:\t", $crontab, "\n";
}


 getCrontabInterval($_SERVER['argv'][1]);

And its output:

serge@serge-laptop:~/www/bin$ php periodical.php '2 hours 25 minutes'
Days:       0
Hours:      2
Minutes:    25
COMMAND:    echo "*/25 */2 * * * %command%" | crontab

So, will the "*/25 */2 * * *" cron value match running command EACH 2 hours 25 minutes? Or it should be something like "0/25 0/2 * * *"? It was not clear for me from manpages. And how to act with days?

UPD: Under each "2 hours 25 minutes" i mean running at 0:00, 2:25, 4:50, 7:15 and etc

SOLUTION: used two-component value for intervals with some manual recommendation to use values like 20 minutes, 3 hours, 4 days and etc.

*/25 */2 * * * means at 0:00, 0:25, 0:50, 2:00, 2:25, 2:50, 4:00, 4:25, 4:50 etc...

25 */2 * * * means 0:25, 2:25, 4:25, 6:25, etc...

0/25 0/2 * * * means 0:25 only

EDIT - After your update

To my knowledge there is no way to do it with a single line in a crontab. The mechanism for specifying at what time your cron job runs is not meant for this kind of complexity. You can however have 9 or 10 entries in your crontab with different times to the same script if you really need that 2h25 functionality

String "*/25 */2 * * *" will runs command each 2 hours EACH 25 minutes
String "25 */2 * * *" will runs command each 2 hours 25 minutes