uasort() Array was modified by the user comparison function - php

Question

I have a class that is basically a wrapper to an Array and implements IteratorAggregate.
When a new object of the class is created, it stores it's value in a protected variable called $value. The value can be a string, integer, etc.. or a traversable object (like an array).
This object is "recursive" since when a traversable object (like an array) is passed to the constructor, he creates another instance of the class. Here's the constructor to clarify:
class ListItem implements \IteratorAggregate
{
protected $readOnly = false;
protected $key;
protected $value;
public function __construct($key, $value, $readOnly = false)
{
if($readOnly) $this->readOnly = true;
if(is_numeric($key)) $key = 'index'.$key;
$this->key = $key;
if (is_array($value) || $value instanceof Traversable) {
$this->value = array();
foreach($value as $k => $v) {
if(is_numeric($k)) $k = 'index'.$k;
$this->value[$k] = new ListItem($k, $v, $readOnly);
}
} else {
$this->value = $value;
}
}
public function __toString()
{
if ( is_array($this->value) ) {
return 'ListItem OBJECT(' . count($this->value) . ')';
} else {
return $this->value;
}
}
Right now, I'm trying to write a simple sorting method for the class.
To sort by Key, this works like a charm:
$success = ksort($this->value, $comparison);
but to sort by value, asort does not work since the actual value I'm trying to sort is stored inside the value property.
So I tried using uasort, like this:
$success = uasort($this->value, function ($a, $b)
{
if ($a->value == $b->value) return 0;
else if($a->value < $b->value) return -1;
else return 1;
});
but for some unclear reason i get the following error:
Warning: uasort() [function.uasort]: Array was modified by the user
comparison function in /* /* /* /ListItem.php on line 129
Q. Why does this happen if I'm just accessing $value for comparison not actually changing anything?

Answer 1

It seems a closure (or anonymous function) is in the global scope which means uasort could not access private or protected members of the ListItem object (although other ListItem Objects can access their sibling's private/protected properties)
this solved the problem: (casting to string)
$success = uasort($this->value, function ($objA, $objB) use ($comparison)
{
$a = (string) $objA;
$b = (string) $objB;
if($comparison == ListItem::SORT_NUMERIC) {
if (is_numeric($a)) $a = (int) $a;
if (is_numeric($b)) $b = (int) $b;
}
if ($a == $b) return 0;
else if($a < $b) return -1;
else return 1;
});