How can I reuse DateTime and DateTimeZone objects in PHP? - php

Question

I made a function to convert datestrings to other timezones, but my function creates new DateTime and DateTimeZone objects every time it's called. This is bad practice so I looked into how I could reuse the objects.
DateTime has a method called setDate, but it requires a year, month and day as arguments instead of a datestring. DateTimeZone has no such method.
Is there a way I can reuse my DateTime and DateTimeZone objects?
Edit: This function will be called anywhere from 4 to hundreds of times in a single POST, depending on how many dates are being returned from the server.
/**
* Converts a datestring to another timezone
*
* #arg $ds (String) - The datestring to be converted
* #arg $tz (String) - The timezone to convert the datestring to
*
* #return (String) - The converted datetime
*/
function convertDateTime($ds, $tz) {
$fooDateTime = new DateTime($ds);
$fooTimeZone = new DateTimeZone($tz);
$newDate = $fooDateTime->setTimezone($fooTimeZone);
return $newDate->format('Y-m-d H:i:s');
}

Answer 1

Compared to the work that is going on in the DateTime and DateTimeZone constructors, the overhead of initialising a new object is miniscule. What is really happening when you call those functions?
The input is being parsed, validated, deconstructed, and in case of DateTimeZone timezone information is being loaded from somewhere.
That data is being stored somewhere (in object properties, but might as well be in variables, an array or whatever else).
You get a handle back to the result of the work which just happened (your new object).
Since every date is unique, this work needs to happen anyway for each date. Whether this is dressed up as an object or as a regular function call which returns an array or something is irrelevant and makes little difference.
Arguably you could memoize the DateTimeZone objects, since identical timezones are identical and immutable anyway. Something like:
static $timezones = [];
if (!isset($timezones[$tz])) {
$timezones[$tz] = new DateTimeZone($tz);
}
But really, for all you know PHP is already optimising this behind the scenes. It probably won't make any appreciable difference in practice.
What you should certainly not do is mutate objects and set entirely new values on them. If you read the list of things that happen above again, it should make sense why; because you don't know what work exactly PHP did there, and you may create inconsistent objects which screw with your business logic due to stupid mistakes.

Answer 2

First of all, confirm it IS a problem. Benchmark your app with current code, then change convertDateTime to return a hardcoded string:
function convertDateTime($ds, $tz) {
return '2016-02-03 15:54:00';
}
and measure the difference.
If there is any, you can probably memoize responses, but it will increase memory consumption, which IMHO is more valuable.
One more thing to keep in mind. One said:
Premature optimization is the root of all evil.
In PHP it is extremely true, as the language itself is far from optimal.

Answer 3

I don't see any bad practice creating multiple DateObject instances, but if you prefer a unique instance, you can extend the DateTime class:
class MyDateTime extends DateTime
{
public function convertDateTime( $ds, $tz )
{
$fooTimeZone = new DateTimeZone( $tz );
$newDate = $this->setTimezone( $fooTimeZone );
return $newDate->format( 'Y-m-d H:i:s' );
}
}
It remains the problem of DateTimeZone class...
Please note that this is only an example to partially answer to your question ( «How can I reuse DateTime» ) without getting into the arguments.
It's easy also add more code to reuse DateTimeZone, i.e. setting an array and then populating it when new TimeZone is called. (Edit: see example in deceze answer)