how to add time/duration from database using php - php

Question

I have this data from data base.
+----+------------+----------+
| id | date time | duration |
+----+------------+----------+-----------
| 3 | 2012-12-20 09:28:53 | ? |
| 1 | 2012-12-20 19:44:10 | ? |
| 2 | 2012-12-23 16:25:15 | |
| 4 | 2012-12-23 18:26:16 | |
| 4 | 2012-12-24 08:01:27 | |
| 5 | 2012-12-29 20:57:33 | |
| 5 | 2012-12-29 20:57:33 | |
+----+------------+----------+------------
duration for id #1 should be equal to the date id #2 - id #1
duration for id #2 should be equal to the date id #3 - id #2
While if the id is the same, it will be added.
Sorry this is only in my mind, still very new in php so I don't know how to start.
Any help is appreciated.
Thanks
edited sorted by date. duration or total time for 1st record = 2nd record - 1st record

Answer 1

If I understand you correctly there is something which is called id=3 and it starts at "2012-12-20 09:28:53" then it ends at "2012-12-20 19:44:10" and in the same second something id=3 starts and you want to know how long everything lasts?
I would do a loop for all records, but I'd start from ending (in SQL: ...ORDER BY date_time DESC), assuming that end of the last (ie. id=5 which started on 2012-12-29 20:57:33) is now, then I calculate duration as substraction of dates (beginning and end), then I would take event beginning as end of the previous one and so on.
An example (not tested):
$end=now();
$dbh=new PDO(...); // here you need to connect to your db, see manual
while($record=$dbh->query('SELECT id, datetime FROM table_name ORDER BY datetime DESC')->fetchObject()){
$start=$record->datetime;
echo $duration=$end-$start; // convert $end and $start to timestamps, if necessary
$end=$start; // here I say that next (in sense of loop, in fact it is previous) record will end at the moment where this record started
}
This doesn't sum because I don't know how are you going to store your data, but I think you will manage with this.
EDITED
At the beginning I define an array:
$durations=array(); // this will hold durations
$ids=array(); // this will hold `id`-s
$last_id=-1; // the value that is not existent
Then the code follows and instead of echo I put this:
$duration=$end-$start;
if($last->id==$record->id){ // is this the same record as before?
$durations[count($durations)-1]->duration+=$duration; // if yes, add to previous value
}
else { // a new id
$durations[]=$duration; // add new duration to array of durations
$ids[]=$record->id; // add new id to array of ids
$last_id=$record->id; // update $last_id
}
and then $end=$start as above.
To view all durations and ids simply
for($i=0;$i<count($durations);$i++){
echo 'id='.$ids[$i].', duration='.$durations[$i].'<br />';
}
Note these tables are in reverse order.