如何在 PHP 中检查变量是否为空

### Answer: Use the PHP empty() function
You can use the PHP `empty()` function to find out whether a [variable](http://www.bixiaguangnian.com/manual/php7/3966.html "variable") is empty or not. A variable is considered empty if it does not exist or if its value equals `FALSE`.

Let's try out the following example to understand how this function basically works:

```php
<?php
$var1 = '';
$var2 = 0;
$var3 = NULL;
$var4 = FALSE;
$var5 = array();
 
// Testing the variables
if(empty($var1)){
    echo 'This line is printed, because the $var1 is empty.';
}
echo "<br>";
 
if(empty($var2)){
    echo 'This line is printed, because the $var2 is empty.';
}
echo "<br>";
 
if(empty($var3)){
    echo 'This line is printed, because the $var3 is empty.';
}
echo "<br>";
 
if(empty($var4)){
    echo 'This line is printed, because the $var4 is empty.';
}
echo "<br>";
 
if(empty($var5)){
    echo 'This line is printed, because the $var5 is empty.';
}
?>
```
<div class="callout callout-ino mb-3">Note: The `empty()` function does not generate a warning if the variable does not exist. That means `empty()` is equivalent to `!isset($var) || $var == false`.</div>