usort overwriting head or tail of array?

I don't fully understand how PHP allocates, so I don't know if I have made an error, or if this is a bug.

usort() here is intermittently zeroing out the head or tail of the array.

class foo
{
开发者_运维问答  public $idx;
  public $data;
}

function compare_foos ($a, $b)
{
  if ($a->data == $b->data)
    return 0;
  elseif ($a->data < $b->data)
    return -1;
  else
    return 1;
}

$mylist = Array ();

for ($i=0; $i < 10; ++$i)
{
  $mylist[$i] = new foo ();
  $mylist[$i]->idx = $i;
  $mylist[$i]->data = rand() % 20;
}

print_r ($mylist);
usort ($mylist, compare_foos);
echo "<HR>";
print_r ($mylist);

Callbacks, like the one used by usort, can be specified in 3 different ways:

• a string containing the name of the function:

  usort($mylist, 'compare_foos');
  

• an actual function:

  usort($mylist, create_function(/*...*/));
  // php >= 5.3 only:
  usort($mylist, function ($a, $b) {
     /* ... */
  });
  

• an array to access object methods:

  usort($mylist, array($myobject, 'mymethod'));
  

Using a callback the way you did is not valid and php returns an error:

PHP Notice:  Use of undefined constant compare_foos - assumed 'compare_foos' in /home/lepidosteus/test.php on line 28

To see it, remember to code with every error enabled on your development machine (but hide them on the production server), using error_reporting():

error_reporting(E_ALL); // display all errors including notices
error_reporting(0); // hide all errors

By the way if you want to use an object as a simple store, you don't need to create a dedicated class you can use stdClass():

$mylist[$i] = new stdClass();
$mylist[$i]->idx = $i;
$mylist[$i]->data = rand() % 20;

Also, you can use var_dump() instead of print_r when you debug to get detailed information about the type of each variable, can help you quickly find out if something is wrong.