About the pit in PHP when the value of a variable is 0

The pitfall when the value of the variable is 0

PHP is a loosely typed language. It has great flexibility in the type, value and operation process of variables. However, with this Flexibility also brings some potential problems. This article mainly discusses the situation in PHP that when a variable is not empty and the value is 0, it is often misjudged as "empty".

In PHP, variables are divided into valued variables and valueless variables. Valued variables refer to variables that have been assigned a value, while valueless variables refer to variables that have not been assigned a value, variables deleted by the unset() function, or variables declared before assignment. For a variable with a value, we can use the isset() function to determine whether the variable exists and return true or false to indicate it.

For example,

$a = 0;
echo isset($a);  // 输出 true

In the above code, although the value of variable $a is 0, because $a exists, the return value of isset($a) is true. In this case, $a is considered a valued variable, not a valueless variable.

When determining whether a variable is empty, we usually use the empty() function. The behavior of the empty() function is somewhat different from the isset() function:

• If the variable does not exist or its value is 0, empty string, null, false or empty array, it returns true. Otherwise return false.
• For string type 0 or numeric type 0, empty() will consider it as a null value and return true.

So, let’s take a look at the following code:

$a = 0;
if (empty($a)) {
    echo '变量为空'；
} else {
    echo '变量不为空'；
}

It seems like a simple code, but its output is somewhat unexpected: the variable is empty.

why? This is because PHP treats a string value of 0 as a null value.

Please see a set of typical examples below.

$a = '';
$b = '0';
$c = null;
$d = false;
$e = 0;
$f = array();

var_dump(empty($a));  // true
var_dump(empty($b));  // true
var_dump(empty($c));  // true
var_dump(empty($d));  // true
var_dump(empty($e));  // true
var_dump(empty($f));  // true

You will find that the value of variable $e is 0, but it is regarded as an empty value by the empty() function. The same problem will also appear in the if() judgment statement:

$a = 0;
if ($a) {
    echo '变量不为空';
} else {
    echo '变量为空';
}

The output result of the above code is that the variable is empty. This is precisely because the value of the variable $a is 0, and 0 is in PHP is considered false.

So, is there any way to avoid this problem? There are two useful functions that can help: is_numeric() and strlen(). The two functions can respectively determine whether a variable is a number and whether the length of the string is 0.

$a = 0;
if ($a !== '' && !is_null($a) && is_numeric($a)) {
    echo '变量不为空';
} else {
    echo '变量为空';
}

The above code uses multiple judgments to ensure that the value of variable $a is indeed not empty, that is, it meets the conditions of not being equal to an empty string, not being a null value, and being a numeric type. This avoids the problems caused by using the empty() function.

In short, in PHP, loose variable types, forced type conversions, valueless variables, and variable values ​​of 0 may cause some problems, and we need to pay special attention to them. When judging variables, it is necessary to choose the correct judgment function and judgment conditions for different situations to avoid pitfalls.

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