Description:
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NaN should never match NaN by definition, but on the Windows builds of 5.2.5, 5.2.6, and 5.2.8 (5.2.7 not tested) it's true.

Strong emphasis on Windows, as it's a Windows-only issue.

Reproduce code:
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$a = acos(8); // returns NaN

var_dump($a == $a); // false on Linux (proper), true on Windows (wrong)

Expected result:
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bool(false)

Actual result:
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bool(true)