找出在数组中添加和删除的项目

I'm building application that tells user which are old and which are new bank notes when I increase sum with X. Everything is fine, but I'm wondering how I can now get list of added and removed items of array?

$old = array(1,5,10);
$new = array(1,5,1);

$added = array_diff($new,$old);
$removed = array_diff($old,$new);

And this is what code above returns:

$added is array(). Incorrect, it should be array([2] => 1).
$removed is array([2] => 10). Correct.

What am I doing wrong, and how can I fix it?

 $added = array_diff($new,$old);

In the above statement, array_diff() compares $new with $old and returns the values in $new that are not present in $old. There is no such value, and hence it returns an empty array.

In short, array_diff() doesn't work with duplicate values. You will have to write a custom function to achieve this. Here's an example:

function array_diff_once($array1, $array2) {
    foreach($array2 as $val) {
        if (false !== ($pos = array_search($val, $array1))) {
            unset($array1[$pos]);
        }    
    }
    return $array1;
}

You can simply use it the same way you did before:

$added   = array_diff_once($new,$old);
$removed = array_diff_once($old,$new);

print_r() of these arrays would correctly output:

Array
(
    [2] => 1
)
Array
(
    [2] => 10
)

Working demo

If you want to check the keys in addition to the values of an array, you should use array_diff_assoc() instead of array_diff():

<?php
    $old = array(1,5,10);
    $new = array(1,5,1);

    $added = array_diff_assoc($new,$old);
    $removed = array_diff_assoc($old,$new);

    echo "<pre>
";     \\ prints array(1) {  [2]=>  int(1) }
    var_dump($added);
    var_dump($removed);
    echo "</pre>
";