array_udiff seems not work - php

Question

I'm trying to compare two array with array_udiff, but it's very weired. it seems array_udiff not get the right answer. Here is the live demo. The result should be an empty array, but leave one element unfiltered out.
<?php
$string = '{
"sakiniai": [
{
"Faktas": "A",
"value": "true"
},
{
"Faktas": "B",
"value": "true"
},
{
"Faktas": "A",
"value": "false"
}
]
}';
$sakiniais = json_decode($string, true)['sakiniai'];
$v = $sakiniais[0];
$arr[] = $v;
$v['value'] = $v['value'] == "true" ? "false" : "true";
$arr[] = $v;
var_dump($arr);
var_dump($sakiniais);
print_r(array_udiff($arr, $sakiniais, function($a, $b){
/*
var_dump($a);
var_dump($b);
var_dump($a == $b);
echo "\n\n\n";
*/
return $a == $b ? 0 : -1;}
));
the output
array(2) {
[0]=>
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(4) "true"
}
[1]=>
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(5) "false"
}
}
array(3) {
[0]=>
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(4) "true"
}
[1]=>
array(2) {
["Faktas"]=>
string(1) "B"
["value"]=>
string(4) "true"
}
[2]=>
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(5) "false"
}
}
Array
(
[1] => Array
(
[Faktas] => A
[value] => false
)
)

Answer 1

The problem is that array_udiff is not performing the comparison between all values, and this seems to be caused by your compare function.
array_udiff() expects that the callable function is a real compare function, but you are returning always 0 and -1, but never 1.
Before doing its job, array_udiff() tries to order both arrays and remove duplicates too. If it can't rely on your comparison function, it can't perform all the needed comparison and some values are "jumped".
Look at all comments in the documentation expecially napcoder comment
Note that the compare function is used also internally, to order the
arrays and choose which element compare against in the next round.
If your compare function is not really comparing (ie. returns 0 if
elements are equals, 1 otherwise), you will receive an unexpected
result.
This is demonstrated looking at your arrays
$arr
array(2) {
[0]=>
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(4) "true"
}
[1]=>
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(5) "false"
}
}
and $sakiniais
array(3) {
[0]=>
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(4) "true"
}
[1]=>
array(2) {
["Faktas"]=>
string(1) "B"
["value"]=>
string(4) "true"
}
[2]=>
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(5) "false"
}
}
You should expect all the combinations between the two arrays to be tested, but the tested combinations (below) doesn't include A-False vs. A-False
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(4) "true"
}
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(5) "false"
}
bool(false)
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(4) "true"
}
array(2) {
["Faktas"]=>
string(1) "B"
["value"]=>
string(4) "true"
}
bool(false)
array(2) {
["Faktas"]=>
string(1) "B"
["value"]=>
string(4) "true"
}
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(5) "false"
}
bool(false)
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(4) "true"
}
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(4) "true"
}
bool(true)
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(4) "true"
}
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(5) "false"
}
bool(false)
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(5) "false"
}
array(2) {
["Faktas"]=>
string(1) "B"
["value"]=>
string(4) "true"
}
bool(false)
In the correct answer at this post there are some other useful insights on how array_udiff works
You can change your return statement like this
if ($a < $b) {
return -1;
} elseif ($a > $b) {
return 1;
} else {
return 0;
}
(if you are wondering how two arrays can be compared for less/greater, have a look at this link, in the section "Comparison with Various Types" and the "Example #2 Transcription of standard array comparison" example)

Answer 2

Nothing strange, everything works as expected. Take a look at arrays comparing:
$arraysAreEqual = ($a == $b); // TRUE if $a and $b have the same key/value pairs.
$arraysAreEqual = ($a === $b); // TRUE if $a and $b have the same key/value pairs in the same order and of the same types.
so, $arr[0] == $sakiniai[0] is true, but $arr[1] == $sakiniai[2] is false and the output is $arr[1]
$arr
array(2) {
[0]=>
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(4) "true"
}
[1]=>
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(5) "false"
}
}
$sakiniai
array(3) {
[0]=>
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(4) "true"
}
[1]=>
array(2) {
["Faktas"]=>
string(1) "B"
["value"]=>
string(4) "true"
}
[2]=>
array(2) {
["Faktas"]=>
string(1) "A"
["value"]=>
string(5) "false"
}
}
result
Array
(
[1] => Array
(
[Faktas] => A
[value] => false
)
)