PHP uksort function using global variable fails after PHP upgrade to 5.3.3

Question

I have a user defined sort function which uses a 'global' declaration in order to refer to a multi-dimensional array when deciding on the correct sort order. It used to work fine under PHP 5.1.6 but now fails under 5.3.3.

The code throws PHP warning:

PHP Warning: uksort(): Array was modified by the user comparison function

But the code definitely does not modify the array.

This code duplicates the problem:

$arr = array();

$arr['i1']['val1'] = 99;
$arr['i1']['val2'] = 100;

$arr['i2']['val1'] = 89;
$arr['i2']['val2'] = 101;


function cmp($a, $b)
{
    global $arr;

    if ($arr[$a]['val2'] > $arr[$b]['val2']) { return 1; }
    if ($arr[$a]['val2'] < $arr[$b]['val2']) { return -1; }
    return 0;
}

if (uksort($arr, 'cmp'))
{
    echo "success";
}
else
{
    echo "failure";
}

Answer 1

If you're not going to sort by the actual keys, don't use uksort but usort or uasort:

function cmp($a, $b) {
    return $a['val2'] - $b['val2'];
}

uasort($arr, 'cmp');

Answer 2

As sidestepping the issue is so popular, here is the cause of your issue:

global $arr;

I'm sure you sensed that aready, to solve that, remove the line and replace $arr with $GLOBALS['arr']. This removes the modification message and you'er accessing the global variable from the symbol table and not the one uksort is currently operating on.

Example:

<?php
$arr = array();

$arr['i1']['val1'] = 99;
$arr['i1']['val2'] = 100;

$arr['i2']['val1'] = 89;
$arr['i2']['val2'] = 101;


function cmp($a, $b)
{
    if ($GLOBALS['arr'][$a]['val2'] > $GLOBALS['arr'][$b]['val2']) { return 1; }
    if ($GLOBALS['arr'][$q]['val2'] < $GLOBALS['arr'][$b]['val2']) { return -1; }
    return 0;
}

if (uksort($arr, 'cmp'))
{
    echo "success\n";
}
else
{
    echo "failure\n";
}

print_r($arr);

Output for 5.3.23 - 5.5.3: (other versions crash/misbehave)

success
Array
(
    [i1] => Array
        (
            [val1] => 99
            [val2] => 100
        )

    [i2] => Array
        (
            [val1] => 89
            [val2] => 101
        )

)

Ups, maybe that's why you see a warning in more recent versions: (Demo: http://3v4l.org/DkK3v)