PHP：使用DateTime-> modify两次，结果相同

Question

I want to have two variables. one -2 seconds of a time and another +2 seconds of the time. This is my code:

$myTime = '2015-08-17 08:19:26';
$myTime = DateTime::createFromFormat('Y-m-d H:i:s', $myTime);

echo '<pre>';

var_dump( $myTime );

$myTime->modify('+2 seconds');
$myTime_p2 = $myTime;
$myTime->modify('-4 seconds');
$myTime_m2 = $myTime;

var_dump( $myTime );
var_dump( $myTime_p2 );
var_dump( $myTime_m2 );

echo '</pre>';

The result is not as expected:

object(DateTime)#1 (3) {
  ["date"]=>
  string(26) "2015-08-17 08:19:26.000000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(16) "America/New_York"
}
object(DateTime)#1 (3) {
  ["date"]=>
  string(26) "2015-08-17 08:19:24.000000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(16) "America/New_York"
}
object(DateTime)#1 (3) {
  ["date"]=>
  string(26) "2015-08-17 08:19:24.000000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(16) "America/New_York"
}
object(DateTime)#1 (3) {
  ["date"]=>
  string(26) "2015-08-17 08:19:24.000000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(16) "America/New_York"
}

As you see the $myTime_p2 and $myTime_m2 variables have the same value. So where is the problem?

Answer 1

This should work:

$myTime->modify('+2 seconds');
$myTime_p2 = clone $myTime;
$myTime->modify('-4 seconds');
$myTime_m2 = clone $myTime;

Answer 2

UPDATE
Using DateTimeImmutable as per @Im0rtality's comment below:

$myTime = '2015-08-17 08:19:26';
$date = DateTimeImmutable::createFromFormat('Y-m-d H:i:s',$myTime);

print $date->modify('+2 seconds')->format('Y-m-d H:i:s') . PHP_EOL;
print $date->modify('-2 seconds')->format('Y-m-d H:i:s') . PHP_EOL;

/*
2015-08-17 08:19:28
2015-08-17 08:19:24
*/

---

You need to clone the initial object to get the two variations

$myTime = '2015-08-17 08:19:26';
$myTime = DateTime::createFromFormat('Y-m-d H:i:s', $myTime);

echo '<pre>';

var_dump( $myTime );

$myTime_p2 = clone $myTime;
$myTime_p2->modify('+2 seconds');

$myTime_m2 = clone $myTime;
$myTime_m2->modify('-4 seconds');

var_dump( $myTime );
var_dump( $myTime_p2 );
var_dump( $myTime_m2 );

echo '</pre>';

<pre>object(DateTime)#1 (3) {
  ["date"]=>
  string(26) "2015-08-17 08:19:26.000000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(13) "Europe/Berlin"
}
object(DateTime)#1 (3) {
  ["date"]=>
  string(26) "2015-08-17 08:19:26.000000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(13) "Europe/Berlin"
}
object(DateTime)#2 (3) {
  ["date"]=>
  string(26) "2015-08-17 08:19:28.000000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(13) "Europe/Berlin"
}
object(DateTime)#3 (3) {
  ["date"]=>
  string(26) "2015-08-17 08:19:22.000000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(13) "Europe/Berlin"
}
</pre>