Why does is_numeric(NAN) return TRUE?

Question

I have tested the is_numeric function in the NAN constant in PHP and the given result

is_numeric(NAN); // TRUE

But NAN means "Not A Number". Why the function is_numeric returns true?

I know that NAN is of float type. But when testing the two cases below, the results are different:

is_float(NAN) // true
filter_var(NAN, FILTER_VALIDATE_FLOAT)// false

Why ocurred it?

_{Sorry, my english is bad}

Answer 1

NAN is a special constant. It has to hold some value, so it holds a float datatype

var_dump(NAN); // float(NAN);

The problem is that filter_var isn't comparing data types, it's looking for numbers, which NAN is not.

Some numeric operations can result in a value represented by the constant NAN. This result represents an undefined or unrepresentable value in floating-point calculations. Any loose or strict comparisons of this value against any other value, including itself, will have a result of FALSE.

Because NAN represents any number of different values, NAN should not be compared to other values, including itself, and instead should be checked for using is_nan().