Mastering PHP’s empty() Function: When Does a Variable Count as Empty?

In PHP programming, checking whether a variable is empty is a common need. The built‑in empty() function is used for this purpose. This article introduces the usage of empty() and provides practical code examples.

The empty() function is simple: it accepts one argument and returns a boolean. It returns true when the argument meets any of the following conditions, otherwise false:

If the variable’s value is 0 or the string "0".

If the variable’s value is false or null.

If the variable is an empty array ( array()) or an object with no properties.

If the variable has never been set.

Example code demonstrating empty() usage:

<?php
$var1 = '';
$var2 = 0;
$var3 = false;
$var4 = null;
$var5 = array();
$var6; // not set

echo 'var1 is empty: ' . (empty($var1) ? 'true' : 'false') . '<br>';
echo 'var2 is empty: ' . (empty($var2) ? 'true' : 'false') . '<br>';
echo 'var3 is empty: ' . (empty($var3) ? 'true' : 'false') . '<br>';
echo 'var4 is empty: ' . (empty($var4) ? 'true' : 'false') . '<br>';
echo 'var5 is empty: ' . (empty($var5) ? 'true' : 'false') . '<br>';
echo 'var6 is empty: ' . (empty($var6) ? 'true' : 'false') . '<br>';
?>

Running the code outputs:

var1 is empty: true
var2 is empty: true
var3 is empty: true
var4 is empty: true
var5 is empty: true
var6 is empty: true

In real applications, empty() is often used to validate form input, for example:

<?php
if (empty($_POST['username'])) {
    echo 'Please enter a username';
} else {
    // other logic
}
?>

Note that empty() can only be used with variables; it cannot directly test constants or expressions. To check a constant, use isset(), and for expression truthiness, use an if statement.

In summary, empty() is a handy PHP function for checking whether a variable is empty, useful for form validation and logical decisions.