It's Lenny's Fault

A very common mistake when first getting in to PHP is to think that isset() and empty() can be used as each others inverse. This is VERY far from the truth and can cause major problems in an application. In this quick article I will explore the differences between isset and empty in PHP

isset

isset will determine if a variable is set and is not NULL. This simply means if the variable has NEVER been set to anything or it is null isset will return false otherwise it will return true. Obviously you can add an ! before it to reverse this depending on it's purpose in your application. Here is a quick example

var_dump(isset($var));   //will output false since it is not set
$var=null;
var_dump(isset($var));   //will output false since it is null
$var=0;
var_dump(isset($var));   //will output true
$var="";
var_dump(isset($var));   //will output true
$var="0";
var_dump(isset($var));   //will output true
$var=array();
var_dump(isset($var));   //will output true
$var=false;
var_dump(isset($var));   //will output true
$var="this is a string";
var_dump(isset($var));   //will output true

empty

empty is quite a bit different than isset. First of all, the most obvious thing is that it works in the opposite way of isset, but there is much more to it than that. Empty will return true for everything isset would return false for PLUS it will also return true for an empty string, an empty array, the string "0″, the number 0, or false. Notice the differences in the same example as above using empty instead of isset.

var_dump(empty($var));   //will output true since it is not set
$var=null;
var_dump(empty($var));   //will output true since it is null
$var=0;
var_dump(empty($var));   //will output true 0 is considered empty
$var="";
var_dump(empty($var));   //will output true "" is an empty string
$var="0";
var_dump(empty($var));   //will output true "0" is empty
$var=array();
var_dump(empty($var));   //will output true for an empty array
$var=false;
var_dump(empty($var));   //will output true since bool false is considered empty
$var="this is a string";
var_dump(empty($var));   //will output false
$var=2;
var_dump(empty($var));   //will output false since 2 is a valid number

As you can see they both have very different results and can clearly NOT be used interchangeably. For a REALLY detailed chart of how different variables test out check out this variable test chart. Enjoy.