In PHP we have three different functions which can be used to test the value of a variable. They are isset(), empty() and is_null(). All these functions return a boolean value. But in case if these functions are not used in correct way they can cause unexpected results.

Isset() - this function is used to check whether a variable is set or not and that is not null, i.e., this function will return boolean true only in a case where variable is not null.

Empty() - this function, as the name suggests, will return boolean true if the variable is an empty string, false, array(), NULL and an unset variable.

is_null() - this function is opposite to the isset() function. This function is used to find whether a variable is NULL. One difference between the isset() and is_null() function is that former can be applied to unknown variables, but is_null() only to declared variables.

Table for reference what these functions will return for different values.

/* Program to show the functionality of isset() */

<?php
	$var= ' ';
	if(isset($var))    // It will evalute to true so the text will be printed.
	{
		echo This variable is set to true.;
	}
	
	$x = sample_data;
	$y = another sample_data;
	var_dump(isset($x));     	  //TRUE
	var_dump(isset($x, $y)); 	 //TRUE
	
	unset($x);

	var_dump(isset($x)); 		//FALSE
	var_dump(isset($x, $y));	//FALSE

	$arr = array('array1' => 1, 'array2' => NULL, 'array3' =>array('sub_array' => 'Sarray'));
	
	var_dump(isset($arr['array1']));	//TRUE
	var_dump(isset($arr['array2']));	//FALSE ; as the key array2 equals NULL so it is 						   considered unset.
	
	var_dump(isset($arr['array3']['sub_array]));		//TRUE

?>

/* Program to demonstrate the comparison between isset() and empty() function */

<?php

	$v = 0;
	
	if(empty($v))			// Evaluates to true as $v is empty
	{
		echo $v is either 0, empty, or is not set;
	}

	if(isset($v))			// Evaluates to true as $v is set
	{
		echo  $v is set and empty ;
	}
?>