Comparing multiple different dates and getting the average in PHP

Question

I use a function called count_days($date1,$date2) that counts the number of days between two dates. But, my question is: the dates come from the DB, in an array like:

Array (
  [imac] => Array (
    [0] => 2002-10-10
    [1] => 2003-11-22
    [3] => 2004-11-10
  )
  [iphone] => Array (
    [0] => 2007-09-11
    [1] => 2008-05-12
    [2] => 2009-06-14
    [3] => 2010-06-05
  )
)

As you can see the products may have a different number of subarrays.
I want to count the days between the first and the next date (and so on!) and then get the average of days.

Answer 1

The DateInterval class is great for this kind of date arithmetic. You can use DateTime::add, DateTime::subtract and DateTime::diff to work with them.

<?php

$types = array(
    'imac' => array ('2002-10-10', '2003-11-22', '2004-11-10'),
    'iphone' => array ( '2007-09-11', '2008-05-12', '2009-06-14', '2010-06-05'),
);

$typeIntervals = array();
$typeAverage = array();
foreach ($types as $type=>$dates) {
    $last = null;
    $typeIntervals[$type] = array();
    foreach ($dates as $date) {
        $current = new DateTime($date);
        if ($last) {
            $interval = $current->diff($last);
            $typeIntervals[$type][] = $interval->days;
        }
        $last = $current;
    }
    $typeAverage[$type] = array_sum($typeIntervals[$type]) / count($typeIntervals[$type]);
}

print_r(typeIntervals);
print_r($typeAverage);

This will output:

Array (
    [imac] => Array (
            [0] => 408
            [1] => 354
        )
    [iphone] => Array (
            [0] => 244
            [1] => 398
            [2] => 356
        )
)
Array (
    [imac] => 381
    [iphone] => 332.66666666667
)