I’ll be honest: most of the posts that I write are either because I’ve solved a problem for a client, or because I solved a problem that Past-david created. This is one of those PD problems, where I wrote some code that stopped functioning. When I looked into it, it turns out that I was using the wrong function to test for a variable in PHP.

There are a variety of functions made to test the state and value of variables, including ones that can tell you if there is anything available to use at all. Three of these functions that are easy to mix up are isset(), empty(), and is_null().

Built-in Variable Testing Tools

All three of these functions are built into PHP, so they should always be available for your use when writing code. empty() and isset() are language constructs, while is_null() is a standard function. We’ll go over why that’s important later in the article.

Before I discuss the difference and show a few examples, here are the descriptions for empty(), isset(), and is_null() from the php.net manual.

empty()

empty ( mixed$var ) : boolCode language: PHP (php)

Determine whether a variable is considered to be empty. A variable is considered empty if it does not exist or if its value equals FALSE. empty() does not generate a warning if the variable does not exist.

isset()

isset ( mixed$var [, mixed$... ] ) : boolCode language: PHP (php)

Determine if a variable is set and is not NULL.

If a variable has been unset with unset(), it will no longer be set. isset() will return FALSE if testing a variable that has been set to NULL. Also note that a null character (“\0”) is not equivalent to the PHP NULL constant.

If multiple parameters are supplied then isset() will return TRUE only if all of the parameters are set. Evaluation goes from left to right and stops as soon as an unset variable is encountered.

is_null()

is_null ( mixed$var ) : boolCode language: PHP (php)

Finds whether the given variable is NULL.

What’s the difference between these variable testing functions?

You can see from the above definitions that these three functions do similar, but not the same things. You’ve gotta determine if you’re trying to test for whether a variable is null, true or false, and whether the variable has been declared.

When to use empty()

If you are using empty() you can test if a variable is false, but also if the variable does not exist. This function is best used when you want to ensure both that the variable exists, and has a value that does not equal false. Note that PHP will treat empty strings, integers of 0, floats of 0.0, empty arrays, and the boolean value of false as false. So basically, only use empty() when you want to ensure that there is some actual value to the variable.

Since you don’t have to declare variables before using them in PHP, you can get in a position where you are trying to perform actions or run other tests on a variable that hasn’t yet been declared. While it’s best practice to declare your variables before use for this and other reasons, this gotcha is one of the reasons that empty() is used differently from isset().

When to use isset()

If you are using isset(), you can test specifically if the variable has been declared already, and that the value is not null. So as long as you have a declared variable that has a value set and is not of the value NULL, you’ll return true when you test it with isset(). This would be a good condition to check before doing other checks to perform actions on a variable:

// Declaring our variable
$variable = 'Some String';

// Testing that our variable exists, then testing the value
if ( isset( $variable ) && $variable !== 'Some Other String' ) {
    echo 'This code evaluates since both of the above are true';
}Code language: PHP (php)

In the above example, we’ve declared our variable as a string, then tested if the variable is set (it is), and if it is not equal to a different string (it is not). Since both of those tests are true, we would then echo out the sentence in that conditional statement.

Should you use is_null()?

Finally, is_null() works in a similar manner to isset() as its opposite, with one key difference: the variable must be declared to return true, provided that it is declared without any value, or is declared specifically as NULL.

I said above that isset() tests whether a variable has been set or not, which is true, but it can handle no variable being set and providing an output of false. That is helpful if somewhere else in the code the unset() construct has been used to remove a variable from scope entirely.

In contrast, is_null() would not only not properly evaluate, it would also return a notice due to its inability to evaluate. Usually that’ll look something like this:

Notice:  Undefined variable: variable in /directory/to/code.php on line XCode language: PHP (php)

Since isset() is both a language construction, and can handle variables that aren’t declared, I’d generally recommend it over using is_null() in any situation. If you need to use is_null(), I might suggest finding a way to rewrite your code instead.

Language Construct vs. Built-In Function

I mentioned before that isset() and empty() are both language constructs in PHP, where is_null() is a built in function. Language constructs are reserved keywords that can evaluate whatever follows them in a specific manner. That means that it already knows what to do without having to find the definition of the construct like it would a function.

The main things to keep in mind between the two when evaluating your code is that language constructs in PHP are slightly faster (but honestly not enough to worry about for speed optimization), they can’t be used in variable functions, and they don’t throw any errors when evaluating variables that don’t exist.

Many times I see warnings and notices because a variable hasn’t been declared, and no one has confirmed that the variable already exists before trying to do some other conditional check with it. Using isset() and empty() can go a long way to avoiding those errors.

Examples of output of these three functions

The following table has been taken directly from a demo created by Virendra Chandak on his personal site. You can view the demo here.

Value of variable ($var)	isset($var)	empty($var)	is_null($var)
“” (an empty string)	bool(true)	bool(true)	bool(false)
” ” (space)	bool(true)	bool(false)	bool(false)
FALSE	bool(true)	bool(true)	bool(false)
TRUE	bool(true)	bool(false)	bool(false)
array() (an empty array)	bool(true)	bool(true)	bool(false)
NULL	bool(false)	bool(true)	bool(true)
“0” (0 as a string)	bool(true)	bool(true)	bool(false)
0 (0 as an integer)	bool(true)	bool(true)	bool(false)
0.0 (0 as a float)	bool(true)	bool(true)	bool(false)
var $var; (a variable declared, but without a value)	bool(false)	bool(true)	bool(true)
NULL byte (“\ 0”)	bool(true)	bool(false)	bool(false)