PHP: isset(), unset() and empty()

PHP has three language constructs to manage variables: isset(), unset() and empty(). We take a look at each of them separately in the below sections.

PHP isset()

The isset() language construct checks whether a certain variable has been previously set or not and returns a boolean value: true if set, false if not or set to null.

php isset unset empty

In the following script, we assign the variable $hello to the string 'HELLO'. Inside the if statement, isset() is applied on $hello to check whether it is set or not 

					
						<?php 
							$hello = 'HELLO';
							if(isset($hello)) {
								echo $hello. ' is set.'; // HELLO is set.
							}
						?>

A variable set to an empty string is also set, and hence, isset() returns true 

					
						$empty = '';
						if(isset($empty)) {
							echo $empty. ' is also set.'; // is also set.
						}

A variable set to a false value is also set, and hence, in this case too isset() returns true 

					
						$untrue = false;
						if(isset($untrue)) {
							echo $untrue. ' is also set.'; // is also set.
						}

Multiple variables can also be passed as arguments to isset(), but it will return true only if all the supplied arguments are set 

					
						$hello = 'HELLO';;
						$empty = '';
						$untrue = false;
						if(isset($hello, $empty, $untrue)) {
							echo $hello, $empty, $untrue, ' are all set.';
						}

PHP unset()

The unset() construct destroys a given variable/set of variables. Let us take the very first example used in the above section and apply unset() on the variable $hello just after it is assigned to 'HELLO'. As we call isset() on the variable $hello, which was just unset(), it returns false 

					
						<?php 
							$hello = 'HELLO';
							unset($hello);
							if(isset($hello)) { // the condition fails; echo is not executed
								echo $hello. ' is set.'; 
							}
						?>

Several variables can be destroyed/unset at a time 

					
						<?php 
							$hello = 'HELLO';
							$n = 1;
							$bool = false;
							unset($hello, $n, $bool);
						?>

PHP empty()

The empty() construct returns true if a variable is set to an empty string, to 0 (either as a number or a string), to false, or to null 

					
						<?php 
							$hello = '';
							$zero = 0;
							$raining = false;
							$nihil = null;
							if(empty($hello)) { // true
								echo 'true'; 
							}
							if(empty($zero)) { // true
								echo 'true'; 
							}
							if(empty($raining)) { // true
								echo 'true'; 
							}
							if(empty($nihil)) { // true
								echo 'true';
							}
						?>

However, a space character is not considered empty. In the following code, we set the variable $space to a single space character. Applying empty() on it, the if condition fails 

					
						<?php 
							$space = ' '; // single space
							if(empty($space)) { // the condition fails; echo is not executed
								echo $space. ' is empty.'; 
							}
						?>