is_null vs empty vs isset... One lesson all PHP coders should learn

PHP has a lot of ways of dealing with variable checking. There are functions that check each type like is_array, is_object or is_bool and there are functions that can be used to check multiple conditions at once. Today we will be dealing with the differences between is_null(), empty() and isset(). First let's explain what each one does.

is_null()

This function checks only whether a variable has a null value, but it doesn't cover for cases when that variable is undefined or for the cases when a value would evaluate to an empty value.

Let's have a look at a few examples:

  
    <?php
    $myvar = NULL; is_null($myvar); // This is TRUE
    $myvar = 0; is_null($myvar); // This is FALSE
    $myvar = FALSE; is_null($myvar); // This is FALSE
    $myvar = '';  is_null($myvar); // This is FALSE
    is_null($some_undefined_var); //This is also TRUE ... but the script throws a notice because $some_undefined_var doesn't exist
  

Please note that a null variable is not the same as an undefined variable. Even if they both evaluate to NULL, PHP will through a notice saying: "Notice: Undefined variable: X in..."

isset()

This function checks whether a variable is defined (a.k.a exists) within your script. However, isset() also sees variables that have the null value as being not set (if you take the function wording literally).

This can be a little confusing but a few examples will show you exactly what I mean:

  
    <?php
    $myvar = NULL; isset($myvar); // This is FALSE
    $myvar = 0; isset($myvar); // This is TRUE
    $myvar = FALSE; isset($myvar); // This is TRUE
    $myvar = '';  isset($myvar); // This is TRUE
    isset($some_undefined_var); // This is FALSE
  

empty()

This function checks whether a variable evaluates to what PHP sees as a "falsy"(a.k.a empty) value. This being said, a variable is empty if it's undefined, null, false, 0 or an empty string.

To better understand the way empty works... you should think about this function as being the same as: !isset($var) || $var==false.

Here are some examples

  
    <?php
    $myvar = NULL; empty($myvar); // This is TRUE
    $myvar = 0; empty($myvar); // This is TRUE
    $myvar = FALSE; empty($myvar); // This is TRUE
    $myvar = '';  empty($myvar); // This is TRUE
    empty($some_undefined_var); // This is TRUE
  

To conclude, remember to ALWAYS do your variable checking. This is especially important in PHP which is not a strictly typed programming language so the programmer needs to pay special attention it.