Description:
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5.5 is wrongly rounded to 5 (even with PHP_ROUND_HALF_UP)

Test script:
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<?php

$n = 2.2459431618637;

echo "Expected 5.5 / is " .
     round(pow(1024, $n - floor($n)), 2, PHP_ROUND_HALF_UP) . "\n";

echo "Expected 5.5 / is " .
     round(pow(1024, $n - floor($n)), 1, PHP_ROUND_HALF_UP) . "\n";

echo "Expected 6   / is " .
     round(pow(1024, $n - floor($n)), 0, PHP_ROUND_HALF_UP) . "\n";

echo "Expected 5.50 / is " . number_format(pow(1024, $n - floor($n)), 2) . "\n";
echo "Expected 5.5  / is " . number_format(pow(1024, $n - floor($n)), 1) . "\n";
echo "Expected 6    / is " . number_format(pow(1024, $n - floor($n)), 0) . "\n";


Expected result:
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See output (line 3 and 6 are wrong):

Expected 5.5 / is 5.5
Expected 5.5 / is 5.5
Expected 6   / is 5
Expected 5.50 / is 5.50
Expected 5.5  / is 5.5
Expected 6    / is 5


Actual result:
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See expected result.