is_nan()

(PHP 4 >= 4.2.0, PHP 5, PHP 7)

判断是否为合法数值

说明

如果$val为“非数值”，例如acos(1.01)的结果，则返回TRUE。

参数

要检查的值

返回值

如果$val不是一个数字（not a number）返回TRUE，否则返回FALSE。

范例

Example #1is_nan()例子

以上例程会输出：

float(NAN)
bool(true)

参见

• is_finite()判断是否为有限值
• is_infinite()判断是否为无限值

nan/"not a number" is not meant to see if the data type is numeric/textual/etc..
NaN is actually a set of values which can be stored in floating-point variables, but dont actually evaluate to a proper floating point number.
The floating point system has three sections: 1 bit for the sign (+/-), an 8 bit exponent, and a 23 bit fractional part.
There are rules governing which combinations of values can be placed into each section, and some values are reserved for numbers such as infinity. This leads to certain combinations being invalid, or in other words, not a number.

I would use is_numeric() instead of ctype_digit() if you cannot be 100% sure what data type the string will be. Example from the docs:

Starting with PHP 7, the string 'NaN' evaluates to the NaN value as well. 
Example:
var_dump( (float) 'NaN' );
PHP 5.x and HHVM:
float(0)
PHP 7.0:
float(NAN)

For seeing whether or not something is a number, use ctype_digit().

I have decided to do some testing because I am getting unusual results with the is_nan function and here are the results of my tests:

Returns:
float(NAN)
bool(true)
Incidentally INF and -INF also evaluate to true.