How to calculate the difference between two dates?

One of the ways to do this object-oriented, is by using the DateTime class , the it has the diff method that returns an object DateInterval , which represents the interval between two distinct dates:

Following the example dates:

$data1 = new DateTime( '2013-12-11' );
$data2 = new DateTime( '1994-04-17' );

$intervalo = $data1->diff( $data2 );

echo "Intervalo é de {$intervalo->y} anos, {$intervalo->m} meses e {$intervalo->d} dias"; 

You can also use the MySQL function DATEDIFF that is quite simple, see the examples:

SELECT DATEDIFF('2013-01-01','2012-03-01')

In the case above, I have passed two dates "manually", they can be two fields:

Imagine that you have a table with the fields data_registro and ultimo_acesso , if you want to show the user the number of days between the registration and the last access, you can use:

SELECT DATEDIFF(ultimo_acesso, data_registro)

If you need to calculate the difference between any date and current date, you can use the NOW () (en)

SELECT DATEDIFF( NOW(), ultimo_acesso)

Solution available from PHP version 5.3+:

$date  = new DateTime('2012-12-25 12:00:00');
$date2 = new DateTime('2013-12-25 12:00:00');

var_dump($date->diff($date2));

Output:

object(DateInterval)[3]
  public 'y' => int 1
  public 'm' => int 0
  public 'd' => int 0
  public 'h' => int 0
  public 'i' => int 0
  public 's' => int 0
  public 'weekday' => int 0
  public 'weekday_behavior' => int 0
  public 'first_last_day_of' => int 0
  public 'invert' => int 0
  public 'days' => int 365
  public 'special_type' => int 0
  public 'special_amount' => int 0
  public 'have_weekday_relative' => int 0
  public 'have_special_relative' => int 0

I'm using php 5.5.7 here, so this output may contain fewer fields if the version is lower.

return date_diff(date_create($data_fim), date_create($data_ini))->format('%d');//days

$resultado = strtotime($data_inicial.' + '.$data_final);