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.
is_nan
(PHP 4 >= 4.2.0, PHP 5, PHP 7, PHP 8)
is_nan — 判断浮点数是否是否为 NAN
说明
返回指定的 num 是否是 NAN（非数值
）。
NAN 是从未定义的数学运算中返回的，例如传递的参数在函数的输入域之外的时候。平方根（sqrt()）仅对正数定义，传递负数将导致
NAN。返回 NAN 的其它操作示例有 INF 除以 INF
以及涉及现有 NAN 值的任何操作。
注意:
尽管名字叫
Not A Number，但NAN是有效的 float 类型。
警告
NAN 不能与 NAN 进行比较。要检测浮点数是否是 NAN，必须使用
is_nan()。使用 $float === NAN 检测将不起作用。
返回值
如果 num 是 NAN，返回 true，否则返回 false。
示例
示例 #1 is_nan() 示例
<?php
$nan = sqrt(-1);
var_dump($nan, is_nan($nan));
?>以上示例会输出：
float(NAN) bool(true)
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User Contributed Notes 7 notes
darkangel at moveinmod dot net ¶
18 years ago
10basetom ¶
9 years ago
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:
<?php
$numeric_string = '42';
$integer = 42;
ctype_digit($numeric_string); // true
ctype_digit($integer); // false (ASCII 42 is the * character)
is_numeric($numeric_string); // true
is_numeric($integer); // true
?>
php at darkain dot com ¶
8 years ago
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)
modern dot apocalypse at gmail dot com ¶
12 years ago
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:
<?php
var_dump(NAN); // float NAN
var_dump(NAN == NAN); // boolean true
var_dump(NAN === NAN); // boolean true
var_dump(is_nan(NAN)); // boolean true
var_dump(NAN == 12); // boolean true
var_dump(NAN === 12); // boolean false
var_dump(is_nan(12)); // boolean false
var_dump(NAN == 12.4); // boolean true
var_dump(NAN === 12.4); // boolean true
var_dump(is_nan(12.4)); // boolean false
var_dump(NAN == NULL); // boolean true
var_dump(NAN === NULL); // boolean false
var_dump(is_nan(NULL)); // boolean false
var_dump(NAN == 'K<WNPO'); // boolean true
var_dump(NAN === 'K<WNPO'); // boolean false
var_dump(is_nan('K<WNPO')); // null and throws a warning "Warning: is_nan() expects parameter 1 to be double, string given in NANTest.php on line 13"
php at keith tyler dot com ¶
14 years ago
It seems odd to me, but in boolean context, NAN evalutes to true.
<?php
var_dump(acos(8));
var_dump((bool)acos(8));
?>
Returns:
float(NAN)
bool(true)
Incidentally INF and -INF also evaluate to true.
