When called inside a function array_multisort changes the global variables instead of the local copy the function should be working with.

<?php

$sortKeys = array(3, 2, 4, 1, 5);
$a = array(111 => '111', 222 => '222', 333 => '333', 444 => '444',
           555 => '555');

echo "initial arrays:<br>";
echo "sortKeys: "; var_dump($sortKeys); echo "<br>";
echo "a: "; var_dump($a); echo "<br>";

echo "<br>";

msortWithRef($sortKeys, $a);

echo "after msortWithRef:<br>";
echo "sortKeys: "; var_dump($sortKeys); echo "<br>";  
echo "a: "; var_dump($a); echo "<br>";  

echo "<br>";

msortWithoutRef($sortKeys, $a);    

echo "after msortWithoutRef:<br>";
echo "sortKeys: "; var_dump($sortKeys); echo "<br>";
echo "a: "; var_dump($a); echo "<br>";

function msortWithoutRef($array1, $array2)
{
    array_multisort($array1, $array2);
}

function msortWithRef($array1, $array2)
{
    array_multisort(&$array1, &$array2);
}

?>

output:

initial arrays:
sortKeys: array(5) { [0]=> int(3) [1]=> int(2) [2]=> int(4) [3]=> int(1) [4]=> int(5) } 
a: array(5) { [111]=> string(3) "111" [222]=> string(3) "222" [333]=> string(3) "333" [444]=> string(3) "444" [555]=> string(3) "555" } 

after msortWithRef:
sortKeys: array(5) { [0]=> int(3) [1]=> int(2) [2]=> int(4) [3]=> int(1) [4]=> int(5) } 
a: array(5) { [111]=> string(3) "111" [222]=> string(3) "222" [333]=> string(3) "333" [444]=> string(3) "444" [555]=> string(3) "555" } 

after msortWithoutRef:
sortKeys: array(5) { [0]=> int(1) [1]=> int(2) [2]=> int(3) [3]=> int(4) [4]=> int(5) } 
a: array(5) { [0]=> string(3) "444" [1]=> string(3) "222" [2]=> string(3) "111" [3]=> string(3) "333" [4]=> string(3) "555" }