Код:
$ php -r 'foreach([INF,+INF,-INF,NAN] as $v) var_dump($v, number_format($v), intval($v)) || print("\n");'
float(INF)
string(3) "inf"
int(0)
float(INF)
string(3) "inf"
int(0)
float(-INF)
string(3) "inf"
int(-9223372036854775808)
float(NAN)
string(3) "nan"
int(-9223372036854775808)