The different behavior of the function uasort in PHP 5.5 and PHP 7.0

Question

I encountered a strange behavior of Magento 1.8 after changing php version from 5.5 to 7.0. This strange behavior is due to a change in the work function uasort.

Source code:

<?php

$arr = [
    "nominal" => [
        "before" => ["subtotal", "grand_total"],
        "after" => [],
        "_code" => "nominal"
    ],
    "subtotal" => [
        "after" => ["nominal"],
        "before" => ["grand_total", "shipping", "freeshipping", "tax_subtotal", "discount", "tax", "weee"],
        "_code" => "subtotal"
    ],
    "shipping" => [
        "after" => ["subtotal", "freeshipping", "tax_subtotal", "nominal", "weee"],
        "before" => ["grand_total", "discount", "tax_shipping", "tax"],
        "_code" => "shipping"
    ],
    "grand_total" => [
        "after" => ["subtotal", "nominal", "shipping", "freeshipping", "tax_subtotal", "discount", "tax"],
        "before" => [],
        "_code" => "grand_total"
    ],
    "msrp" => [
        "before" => [],
        "after" => [],
        "_code" => "msrp"
    ],
    "freeshipping" => [
        "after" => ["subtotal", "nominal"],
        "before" => ["tax_subtotal", "shipping", "grand_total", "tax", "discount"],
        "_code" => "freeshipping"
    ],
    "discount" => [
        "after" => ["subtotal", "shipping", "nominal", "freeshipping", "tax_subtotal", "tax_shipping", "weee"],
        "before" => ["grand_total", "tax"],
        "_code" => "discount"
    ],
    "tax_subtotal" => [
        "after" => ["0" => "freeshipping", "1" => "subtotal", "3" => "nominal"],
        "before" => ["tax", "discount", "shipping", "grand_total", "tax_shipping", "weee"],
        "_code" => "tax_subtotal"
    ],
    "tax_shipping" => [
        "after" => ["shipping", "tax_subtotal", "subtotal", "freeshipping", "nominal"],
        "before" => ["tax", "discount", "grand_total"],
        "_code" => "tax_shipping"
    ],
    "tax" => [
        "after" => ["subtotal", "shipping", "discount", "tax_subtotal", "freeshipping", "tax_shipping", "nominal", "weee"],
        "before" => ["grand_total"],
        "_code" => "tax"
    ],
    "weee" => [
        "after" => ["subtotal", "tax_subtotal", "nominal", "freeshipping"],
        "before" => ["shipping", "tax", "discount", "grand_total", "tax_shipping"],
        "_code" => "weee"
    ]
];


function _compareTotals($a, $b)
{
    $aCode = $a['_code'];
    $bCode = $b['_code'];
    if (in_array($aCode, $b['after']) || in_array($bCode, $a['before'])) {
        $res = -1;
    } elseif (in_array($bCode, $a['after']) || in_array($aCode, $b['before'])) {
        $res = 1;
    } else {
        $res = 0;
    }
    echo sprintf("%s <> %s: %s", $aCode, $bCode, $res) . "\n";
    return $res;
}

uasort($arr, '_compareTotals');
var_dump(array_keys($arr));

In php 5.5 result is:

freeshipping <> subtotal: 1
freeshipping <> shipping: -1
weee <> freeshipping: 1
tax <> freeshipping: 1
tax_shipping <> freeshipping: 1
tax_subtotal <> freeshipping: 1
discount <> freeshipping: 1
nominal <> freeshipping: -1
freeshipping <> grand_total: -1
msrp <> freeshipping: 0
subtotal <> msrp: 0
nominal <> subtotal: -1
tax_subtotal <> shipping: -1
weee <> tax_subtotal: 1
tax <> tax_subtotal: 1
tax_shipping <> tax_subtotal: 1
grand_total <> tax_subtotal: 1
discount <> tax_subtotal: 1
shipping <> tax_subtotal: 1
grand_total <> discount: 1
grand_total <> shipping: 1
grand_total <> tax_shipping: 1
grand_total <> tax: 1
weee <> grand_total: -1
shipping <> discount: -1
tax <> shipping: 1
tax_shipping <> shipping: 1
weee <> shipping: -1
tax_shipping <> discount: -1
tax <> tax_shipping: 1
discount <> tax_shipping: 1
tax <> discount: 1

array(11) {
  [0] =>
  string(7) "nominal"
  [1] =>
  string(8) "subtotal"
  [2] =>
  string(4) "msrp"
  [3] =>
  string(12) "freeshipping"
  [4] =>
  string(12) "tax_subtotal"
  [5] =>
  string(4) "weee"
  [6] =>
  string(8) "shipping"
  [7] =>
  string(12) "tax_shipping"
  [8] =>
  string(8) "discount"
  [9] =>
  string(3) "tax"
  [10] =>
  string(11) "grand_total"
}

In php 7.0 result is:

nominal <> subtotal: -1
subtotal <> shipping: -1
shipping <> grand_total: -1
grand_total <> msrp: 0
msrp <> freeshipping: 0
freeshipping <> discount: -1
discount <> tax_subtotal: 1
msrp <> tax_subtotal: 0
freeshipping <> tax_subtotal: -1
discount <> tax_shipping: 1
freeshipping <> tax_shipping: -1
tax_subtotal <> tax_shipping: -1
discount <> tax: -1
tax <> weee: 1
tax_shipping <> weee: 1
freeshipping <> weee: -1
tax_subtotal <> weee: -1

array(11) {
  [0] =>
  string(7) "nominal"
  [1] =>
  string(8) "subtotal"
  [2] =>
  string(8) "shipping"
  [3] =>
  string(11) "grand_total"
  [4] =>
  string(4) "msrp"
  [5] =>
  string(12) "freeshipping"
  [6] =>
  string(12) "tax_subtotal"
  [7] =>
  string(4) "weee"
  [8] =>
  string(12) "tax_shipping"
  [9] =>
  string(8) "discount"
  [10] =>
  string(3) "tax"
}

In PHP5 grand_total is the last element, but in PHP7 - no. The problem is related to the uncertainty of the location msrp element. I found a research on this subject associated with php 5.

I solved this problem by indicating a relative position msrp. But I wonder why it works in php5 and does not work in php7. These are the features of the new version of php or a bug?

addition #1

The problem not only in that the PHP7 don't knows how to sort equal elements, for example msrp and grand_total. If you look at item shipping and freeshipping, then they are clearly defined, who should be earlier. PHP5 solves this problem, and PHP7 not.

Answer 1

From the usort() documentation:

Note: If two members compare as equal, their relative order in the sorted array is undefined.

This is what you're seeing here. PHP 7 uses a different, partially stable sorting algorithm, so elements that compare equal according to your sorting function may now have a different order.

If you care about the sorting order of equal elements (and this is not just a testing issue), you should make it explicit in your comparison function.