Why is it possible that is_numeric returns false when is_nan returns true?

In PHP, is_nan() and is_numeric() are two commonly used functions, which are used to determine whether a variable is "not a number" (NaN) and whether it is a number or a numeric string. However, many people may be confused why in some cases is_nan() returns true , is_numeric() may return false . This article will dig into how these two functions work in depth and explain the differences between them.

1. How is the working principle of is_nan() function

is_nan() is a function used to check whether a value is NaN (Not a Number). NaN is a special floating point value that represents an illegal numerical calculation result. For example, when you perform some math operations, NaN may be returned if the result is not a valid number. Generally, NaN is caused by the following situations:

• 0 Divided by 0

• Square root of negative number

• Invalid mathematical expression (such as: converting invalid strings into numbers)

These illegal numbers can be detected using is_nan() .

Example:

 $var1 = acos(2); // This is an illegal operation，acos(2) Will return NaN
$var2 = 0 / 0;   // 0 Divide by 0 The result is also NaN

echo is_nan($var1); // Output true
echo is_nan($var2); // Output true

2. How is the working principle of is_numeric() function

is_numeric() is used to determine whether a variable is a numeric string or a numeric string. This function checks whether the given value is a legal numeric type, or whether it can be converted to a number. It should be noted that is_numeric() does not check whether the value is a valid floating point number, but only checks whether it can be represented as a number (including integers or floating point number).

Example:

 echo is_numeric(123);     // Output true，123 It's a number
echo is_numeric("123");   // Output true，String "123" 也It's a number
echo is_numeric("12.34"); // Output true，String "12.34" 也It's a number
echo is_numeric("abc");   // Output false，"abc" 不It's a number

3. Why do is it different return values ​​for is_nan() and is_numeric() ?

Although both is_nan() and is_numeric() involve detection of numbers, their inspection mechanisms vary, especially when dealing with NaN.

• is_nan() is specifically used to check if a value is NaN, which is a special "non-number" value. NaN is a state of floating points, and is_numeric() only cares whether the value can be converted into a valid number.

• When is_nan() returns true , is_numeric() returns false because NaN is not a valid number, and is_numeric() can only recognize valid numbers or strings that can be converted into numbers. NaN cannot be considered a legal number, so is_numeric() returns false .

Example:

 $var = acos(2); // NaN

echo is_nan($var);    // Output true，$var yes NaN
echo is_numeric($var); // Output false，NaN 不yes一个合法的数字

4. Summary

• is_nan() can only determine whether a value is NaN. When it returns true , it means that the value is an illegal number.

• is_numeric() is used to check if a value can be used as a valid number, while NaN is considered an invalid number, so it returns false .

This is why in PHP, is_numeric() returns false when is_nan() returns true .