PHP has different functions which can be used to test the value of a variable. Three useful functions for this are isset(), empty() and is_null(). All these function return a boolean value. If these functions are not used in correct way they can cause unexpected results.
isset() and empty() are often viewed as functions that are opposite, however this is not always true. In this post I will explain the differences between these functions.
isset()
isset — Determine if a variable is set and is not NULL
In other words, it returns true only when the variable is not null.
empty()
empty — Determine whether a variable is empty
In other words, it will return true if the variable is an empty string, false, array(), NULL, “0?, 0, and an unset variable.
is_null()
is_null — Finds whether a variable is NULL
In other words, it returns true only when the variable is null. is_null() is opposite of isset(), except for one difference that isset() can be applied to unknown variables, but is_null() only to declared variables.
| Value of variable ($var) | isset($var) | empty($var) | is_null($var) |
|---|---|---|---|
| “” (an empty string) | bool(true) | bool(true) | |
| ” ” (space) | bool(true) | ||
| FALSE | bool(true) | bool(true) | |
| TRUE | bool(true) | ||
| array() (an empty array) | bool(true) | bool(true) | |
| NULL | bool(true) | bool(true) | |
| “0? (0 as a string) | bool(true) | bool(true) | |
| 0 (0 as an integer) | bool(true) | bool(true) | |
| 0.0 (0 as a float) | bool(true) | bool(true) | |
| var $var; (a variable declared, but without a value) | bool(true) | bool(true) | |
| NULL byte (“\ 0?) | bool(true) |
- Wampserver 2.1, Apache 2.2.21, PHP 5.3.8
- Wampserver 2.1, Apache 2.2.17, PHP 5.3.5
- Wampserver 2.1, Apache 2.2.17, PHP 5.2.11
You can use the code below to get the above table.
<?php
echo '<table border="1">';
echo '<tr><th>Value of variable ($var)</th><th>isset($var)</th><th>empty($var)</th><th>is_null($var)</th></tr>';
$var = '';
echo '<tr><td>"" (an empty string)</td><td>';
var_dump(isset($var));
echo '</td><td>';
var_dump(empty($var));
echo '</td><td>';
var_dump(is_null($var));
echo '</td></tr>';
$var = ' ';
echo '<tr><td>" " (space)</td><td>';
var_dump(isset($var));
echo '</td><td>';
var_dump(empty($var));
echo '</td><td>';
var_dump(is_null($var));
echo '</td></tr>';
$var = FALSE;
echo '<tr><td>FALSE</td><td>';
var_dump(isset($var));
echo '</td><td>';
var_dump(empty($var));
echo '</td><td>';
var_dump(is_null($var));
echo '</td></tr>';
$var = TRUE;
echo '<tr><td>TRUE</td><td>';
var_dump(isset($var));
echo '</td><td>';
var_dump(empty($var));
echo '</td><td>';
var_dump(is_null($var));
echo '</td></tr>';
$var = array();
echo '<tr><td>array() (an empty array)</td><td>';
var_dump(isset($var));
echo '</td><td>';
var_dump(empty($var));
echo '</td><td>';
var_dump(is_null($var));
echo '</td></tr>';
$var = NULL;
echo '<tr><td>NULL</td><td>';
var_dump(isset($var));
echo '</td><td>';
var_dump(empty($var));
echo '</td><td>';
var_dump(is_null($var));
echo '</td></tr>';
$var = '0';
echo '<tr><td>"0" (0 as a string)</td><td>';
var_dump(isset($var));
echo '</td><td>';
var_dump(empty($var));
echo '</td><td>';
var_dump(is_null($var));
echo '</td></tr>';
$var = 0;
echo '<tr><td>0 (0 as an integer)</td><td>';
var_dump(isset($var));
echo '</td><td>';
var_dump(empty($var));
echo '</td><td>';
var_dump(is_null($var));
echo '</td></tr>';
$var = 0.0;
echo '<tr><td>0.0 (0 as a float)</td><td>';
var_dump(isset($var));
echo '</td><td>';
var_dump(empty($var));
echo '</td><td>';
var_dump(is_null($var));
echo '</td></tr>';
unset($var); // doing this just as a precaution, to make sure $var is actually not defined.
$var;
echo '<tr><td>var $var; (a variable declared, but without a value)</td><td>';
var_dump(isset($var));
echo '</td><td>';
var_dump(empty($var));
echo '</td><td>';
var_dump(is_null($var));
echo '</td></tr>';
$var = '\0';
echo '<tr><td>NULL byte ("\ 0")</td><td>';
var_dump(isset($var));
echo '</td><td>';
var_dump(empty($var));
echo '</td><td>';
var_dump(is_null($var));
echo '</td></tr>';
echo '</table>';
?>