Correctly handing NAN in PHP versions

Can anyone explain why NAN and a variable equal to NAN behave differently depending on the version of PHP?

Consider the following code:

$nan = NAN;
print "PHP Version: " . phpversion(). "\n" .
  '0 <  NAN ? ' . ( 0 < NAN   ? 'TRUE' : 'FALSE' ) . "\n" .
  '0 >  NAN ? ' . ( 0 > NAN   ? 'TRUE' : 'FALSE' ) . "\n" . 
  '0 == NAN ? ' . ( 0 == NAN  ? 'TRUE' : 'FALSE' ) . "\n" . 
  '0 <  $nan ? ' . ( 0 < $nan  ? 'TRUE' : 'FALSE' ) . "\n" .
  '0 >  $nan ? ' . ( 0 > $nan  ? 'TRUE' : 'FALSE' ) . "\n" .
  '0 == $nan ? ' . ( 0 == $nan ? 'TRUE' : 'FALSE' ) . "\n" .
  'is_nan(NAN) ' . ( is_nan(NAN)  ? 'TRUE' : 'FALSE' ) . "\n" .
  'is_nan($nan) ' . ( is_nan($nan) ? 'TRUE' : 'FALSE' ) . "\n" .
  'gettype(NAN) is '  . gettype(NAN)  . "\n" .
  'gettype($nan) is ' . gettype($nan) . "\n";

Now, if I run this code against many versions of PHP (using MAMP) the following are results:

PHP Version: 5.3.5
0 <  NAN ? TRUE
0 >  NAN ? TRUE
0 == NAN ? FALSE
0 <  $nan ? TRUE
0 >  $nan ? TRUE
0 == $nan ? FALSE
is_nan(NAN) TRUE
is_nan($nan) TRUE
gettype(NAN) is double
gettype($nan) is double

PHP Version: 5.6.30 (and 5.5.30, 5.4.45)
0 <  NAN ? FALSE
0 >  NAN ? FALSE
0 == NAN ? FALSE
0 <  $nan ? FALSE
0 >  $nan ? FALSE
0 == $nan ? FALSE
is_nan(NAN) TRUE
is_nan($nan) TRUE
gettype(NAN) is double
gettype($nan) is double

PHP Version: 7.1.1 (and 7.0.15)
0 <  NAN ? TRUE
0 >  NAN ? TRUE
0 == NAN ? FALSE
0 <  $nan ? FALSE
0 >  $nan ? FALSE
0 == $nan ? FALSE
is_nan(NAN) TRUE
is_nan($nan) TRUE
gettype(NAN) is double
gettype($nan) is double

Can a function in PHP rely on a comparison to NAN or should NAN only be used with is_nan()?