Learn to use PHP empty() function to determine whether a variable is empty

PHP empty() function is a very commonly used function, used to determine whether a variable is empty. In PHP programming, using the empty() function can very conveniently determine whether a variable is empty to avoid errors during program runtime.

First of all, the empty() function has the following characteristics:

1. The empty() function can accept one parameter, which is the variable or expression to be judged.
2. The empty() function returns a Boolean value, if the variable is empty, it returns true, otherwise it returns false.

3. The following variables are considered "empty" (return true):

   • A variable that has not been assigned a value
   • A variable that has been assigned a value of NULL
   • A variable assigned to the empty string ""
   • A variable assigned to 0 or the string "0"
   • A variable assigned to false

Below we use examples to illustrate how to use the empty() function.

Example 1: Determine whether a variable that has not been assigned a value is empty

<?php
    $var;
    if (empty($var)) {
        echo "该变量没有被赋值";
    } else {
        echo "该变量被赋值了";
    }
?>

The output result is: the variable has not been assigned a value.

Explanation: The variable $var has not been assigned a value, so it is considered empty and returns true.

Example 2: Determine whether a variable assigned to NULL is empty

<?php
    $var = NULL;
    if (empty($var)) {
        echo "该变量被赋值为NULL";
    } else {
        echo "该变量不是NULL";
    }
?>

The output result is: the variable is assigned to NULL.

Explanation: The variable $var is assigned a value of NULL, so it is considered empty and returns true.

Example 3: Determine whether a variable assigned to the empty string "" is empty

<?php
    $var = "";
    if (empty($var)) {
        echo "该变量被赋值为空字符串";
    } else {
        echo "该变量不是空字符串";
    }
?>

The output result is: the variable is assigned to the empty string.

Explanation: The variable $var is assigned an empty string, so it is considered empty and returns true.

Example 4: Determine whether a variable assigned a value of 0 or the string "0" is empty

<?php
    $var1 = 0;
    $var2 = "0";
    if (empty($var1)) {
        echo "该变量被赋值为0";
    } else {
        echo "该变量不是0";
    }
    if (empty($var2)) {
        echo "<br>该变量被赋值为字符串0";
    } else {
        echo "<br>该变量不是字符串0";
    }
?>

The output result is: the variable is assigned a value of 0
The variable is assigned a value is string 0.

Explanation: The variable $var1 is assigned a value of 0, and the variable $var2 is assigned a value of "0". They are both regarded as empty and return true respectively.

Example 5: Determine whether a variable assigned to false is empty

<?php
    $var = false;
    if (empty($var)) {
        echo "该变量被赋值为false";
    } else {
        echo "该变量不是false";
    }
?>

The output result is: the variable is assigned to false.

Explanation: The variable $var is assigned false, so it is considered empty and returns true.

Summary: The empty() function can help us determine whether the variable is empty and avoid errors when the program is running. When using the empty() function, you need to pay attention to the type and assignment of variables to avoid unexpected results.

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