Description:
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The (float) typecast operator applied to a string is expected to return the floating point number represented by the string, instead the special values "NAN", "INF" and "-INF" are not handled properly and give 0.0, 0.0 and -0.0 respectively.

To make the (float) operator applied to a string symmetrical versus the (string) operator applied to a float, the special strings "NAN", "INF" and "-INF" case sensitive should be translated into NAN, INF and -INF respectively:

$f = NAN;
$s = (string) $f;  # ==> "NAN"
$g = (float) $s; # ==> NAN again

Apparently the serialize/unserialize process works properly as it was fixed in bug #27646:

var_dump( unserialize(serialize(NAN)) ) ==> float(NAN)

Reproduce code:
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var_dump( (float) "INF" ); # expected float(INF)
var_dump( (float) "-INF" ); # expected float(-INF)
var_dump( (float) "NAN" ); # expected float(NAN)


Expected result:
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float(INF)
float(-INF)
float(NAN)


Actual result:
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float(0)
float(-0)
float(0)