PHP的DateTime数组似乎有些偏差

Question

I am trying to create a function to create an array of DateTime(s). The dates I want are every Saturday in a given year.

I have a function that does this but when the values are stored in an array, they begin with the second Saturday of the year and extend to the first Saturday of the following year.

Note that the following displays a list of Saturdays as generated by the function and then after a couple of blank lines displays the Saturdays store in the array.

<?php
define ('sql','Y-m-d');
define ('br','<br/>');

function allSaturdays ($year){
    $endofyear = "$year-12-31";
    $interval = new DateInterval("P7D");

    $year--;
    $workingdate = "$year-12-31";  

    $workingdate  = strtotime ($workingdate);
    $workingdate  = strtotime ("next Saturday",$workingdate); 

    $workingdate  = date ("Y-m-d",$workingdate);   
    $workingdate  = new DateTime ($workingdate); 
    $result[] = new DateTime;
    while ($workingdate->format(sql) <= $endofyear ) {
       $result[] = $workingdate;
       echo $workingdate->format (sql).br;     
       $workingdate->add ($interval);
       $workingdate = new DateTime ($workingdate->format(sql));
       //echo $workingdate->format (sql)."#".br;;
    }// while
      unset ($workingdate);
      return $result;  

 }//function


  $sats = allSaturdays(2016);

  echo "<br/.<br/>";
  foreach ($sats as $saturday)
     echo $saturday->format(sql)."*<br>";


  ?>

However, if I store the only the date (2016-01-02) the correct values are in the array.

No doubt I'm missing something simple. Any help is appreciated.

Dave

Answer 1

You are modifying your DateTime object after you placed it in the array.

This is where you add it to the array:

$result[] = $workingdate;

And then the line after that adds 7 days to it:

$workingdate->add($interval);

Calling add modifies the original. If you want to make sure you're working on a copy, call clone first or use DateTimeImmutable instead of DateTime.

Here's a new version of your code using DateTimeImmutable:

function allSaturdays ($year){

    $endOfYear = new DateTimeImmutable("$year-12-31");
    $workingdate = new DateTimeImmutable("first saturday of January " . $year);

    while ($workingdate <= $endofyear ) {
       $result[] = $workingdate;
       $workingDate = $workingDate->modify('+7 days');
    }// while
    return $result;  

 }//function

Here's another version that does not use DateTimeImmutable

function allSaturdays ($year){

    $endOfYear = new DateTime("$year-12-31");
    $workingdate = new DateTime("first saturday of January " . $year);

    while ($workingdate <= $endofyear ) {
       $result[] = $workingdate;
       $workingDate = clone $workingDate;
       $workingDate->modify('+7 days');
    }// while
    return $result;  

 }//function