Description:
------------
All versions of PHP seem to be inefficient at executing array_udiff. The values in $a in the callback function are all compared twice. If the callback function is expensive, this would affect performance.

Also, some comparisons are unnecessary.

Test script:
---------------
/* Note: The callback function just returns 0 every time, which means that all values in both arrays are considered equal. */

function callback($a, $b) {
    echo "\$a = $a; \$b= $b; ";
    echo "<br>\n";
    return 0;
}

$array1 = array("first_0", "first_1", "first_2", "first_3", "first_4" );
$array2 = array("second_0", "second_1", "second_2", "second_3", "second_4");
$result = array_udiff($array1, $array2, "callback");

Actual result:
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$a = first_2; $b= first_1;
$a = first_4; $b= first_2;
$a = first_2; $b= first_0;
$a = first_3; $b= first_2;
$a = first_4; $b= first_3; // Unnecessary since we already know these are equal
$a = first_1; $b= first_0; // Unnecessary since we already know these are equal
$a = second_2; $b= second_1;
$a = second_4; $b= second_2;
$a = second_2; $b= second_0;
$a = second_3; $b= second_2;
$a = second_4; $b= second_3; // Unnecessary since we already know these are equal
$a = second_1; $b= second_0; // Unnecessary since we already know these are equal
$a = first_4; $b= second_4;
$a = first_4; $b= first_3; // Duplicate. Already tested above.
$a = first_3; $b= first_2; // Duplicate. Already tested above.
$a = first_2; $b= first_1; // Duplicate. Already tested above.
$a = first_1; $b= first_0; // Duplicate. Already tested above.