sortBy() function for PHP (inspired by Scala)

sortBy.php

<?php
/**
 * Sorts an array based on keys provided by a closure which extracts a value from the array's elements.
 *
 * Keys are not preserved.  The original is not modified.
 *
 * Inspired by Scala's Seq.sortBy()
 * @see http://www.scala-lang.org/api/current/index.html#scala.collection.Seq
 *
 * @param $array array-like To sort. (Really it just needs to be something iterable.)
 * @param $descendingOrder boolean Whether the keys should be sorted in ascending or descending order.
 * @param $keyExtractor Closure Function that will be passed each element of $array to produce a key for sorting. Must return an int or string.
 *
 * @returns a new array sorted as you requested.
 */
public static function sortBy($array, $descendingOrder, \Closure $keyExtractor)
{
    $newArray = array();
    foreach ($array as $value) {
        $newArray[] = $value;
    }
    $descendingMultiplier = $descendingOrder ? -1 : 1;
    $usortSucceeded = usort($newArray, function($a, $b) use ($descendingMultiplier, $keyExtractor) {
        $aKey = $keyExtractor($a);
        $bKey = $keyExtractor($b);
        if (is_int($aKey) && is_int($bKey)) {
            return $descendingMultiplier * (($aKey < $bKey) ? (-1) : (($aKey === $bKey) ? (0) : (1)));
        }
        if (is_string($aKey) && is_string($bKey)) {
            return $descendingMultiplier * strcmp($aKey, $bKey);
        }
        throw new \Exception("keyExtractor must return an int or string.");
    });
    if (!$usortSucceeded) {
        throw new \Exception("Unable to sort array.");
    }
    return $newArray;
}

function testSortBy() {
    $a = array(
        2 => array('a' => 2),
        1 => array('a' => 1),
        0 => array('a' => 3),
    );
    $sorted = Framework::sortBy($a, false, function($e) { return $e['a']; });
    $this->assertTrue(is_array($sorted));
    $this->assertIdentical(count($sorted), count($a));
    $this->assertIdentical(reset($sorted), $a[1]);
    $this->assertIdentical(next($sorted), $a[2]);
    $this->assertIdentical(next($sorted), $a[0]);
}

function testSortByReverse() {
    $a = array(
        2 => array('a' => 2),
        1 => array('a' => 1),
        0 => array('a' => 3),
    );
    $sorted = Framework::sortBy($a, true, function($e) { return $e['a']; });
    $this->assertTrue(is_array($sorted));
    $this->assertIdentical(count($sorted), count($a));
    $this->assertIdentical(reset($sorted), $a[0]);
    $this->assertIdentical(next($sorted), $a[2]);
    $this->assertIdentical(next($sorted), $a[1]);
}

function testSortByDuplicateKeys() {
    $a = array(
        3 => array('a' => 2),
        2 => array('a' => 2),
        1 => array('a' => 1),
        0 => array('a' => 3),
    );
    $sorted = Framework::sortBy($a, false, function($e) { return $e['a']; });
    $this->assertTrue(is_array($sorted));
    $this->assertIdentical(count($sorted), count($a));
    $this->assertIdentical(reset($sorted), $a[1]);
    $this->assertIdentical(next($sorted), $a[3]);
    $this->assertIdentical(next($sorted), $a[2]);
    $this->assertIdentical(next($sorted), $a[0]);
}

function testSortByWithStringKeys() {
    $a = array(
        3 => array('a' => 'B'),
        2 => array('a' => 'B'),
        1 => array('a' => 'A'),
        0 => array('a' => 'C'),
    );
    $sorted = Framework::sortBy($a, false, function($e) { return $e['a']; });
    $this->assertTrue(is_array($sorted));
    $this->assertIdentical(count($sorted), count($a));
    $this->assertIdentical(reset($sorted), $a[1]);
    $this->assertIdentical(next($sorted), $a[3]);
    $this->assertIdentical(next($sorted), $a[2]);
    $this->assertIdentical(next($sorted), $a[0]);
}