Isset, empty, and is_null behavior in PHP - Step-by-Step Execution

Concept Flow - Isset, empty, and is_null behavior

Start

↓

Check isset(var)

Yes→isset = true

↓

Check empty(var)

Yes→empty = true

↓

Check is_null(var)

Yes→is_null = true

↓

is_null = false

↓

End

The flow checks if a variable is set, then if it is empty, and finally if it is null, showing how each function behaves step-by-step.

Execution Sample

PHP

<?php
$a = null;
$b = 0;
$c = '';
$d = 'hello';

var_dump(isset($a));
var_dump(empty($a));
var_dump(is_null($a));
?>

This code tests isset, empty, and is_null on a variable set to null.

Execution Table

Step	Variable	isset()	empty()	is_null()	Explanation
1	$a = null	false	true	true	$a is null, so isset is false, empty is true, is_null is true
2	$b = 0	true	true	false	$b is zero, isset true, empty true (0 is empty), is_null false
3	$c = ''	true	true	false	$c is empty string, isset true, empty true, is_null false
4	$d = 'hello'	true	false	false	$d is non-empty string, isset true, empty false, is_null false
5	$e not set	false	true	false	$e is not set, isset false, empty true, is_null false

💡 All variables checked; isset false if variable not set or null; empty true for null, 0, '', false; is_null true only if variable is null.

Variable Tracker

Variable	Initial	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
$a	undefined	null	null	null	null	null
$b	undefined	undefined	0	0	0	0
$c	undefined	undefined	undefined	''	''	''
$d	undefined	undefined	undefined	undefined	'hello'	'hello'
$e	undefined	undefined	undefined	undefined	undefined	undefined

Key Moments - 3 Insights

Why does isset($a) return false when $a is null?

Because isset returns false if the variable is null or not set, as shown in execution_table row 1.

Why is empty($b) true when $b is 0?

Because empty considers 0 as empty (falsey), so empty returns true, as shown in execution_table row 2.

Why does is_null($e) return false even though $e is not set?

Because is_null returns false if the variable is not set, as shown in execution_table row 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the result of isset($c) at step 3?

A true

B false

C null

D error

💡 Hint

Check the 'isset()' column for $c at step 3 in the execution_table.

At which step does empty() return false?

A Step 1

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'empty()' column in the execution_table and find where it is false.

If $a was set to false instead of null, what would isset($a) return?

A false

B true

C null

D error

💡 Hint

Recall that isset returns true for variables set to false, only false for null or unset variables.

Concept Snapshot

isset(var): true if var exists and is not null
empty(var): true if var is falsey (null, 0, '', false, etc.)
is_null(var): true if var is null
isset false if var is null or unset
empty true for many falsey values
is_null true only for null

Full Transcript

This visual execution shows how PHP functions isset, empty, and is_null behave with different variable values. isset returns false if the variable is null or not set, true otherwise. empty returns true for variables that are falsey like 0, empty string, null, or false. is_null returns true only if the variable is null. The execution table traces these functions step-by-step on variables with values null, 0, empty string, a non-empty string, and an unset variable. The variable tracker shows how variables change or remain undefined. Key moments clarify common confusions like why isset returns false for null and why empty returns true for 0. The quiz tests understanding by asking about specific steps and hypothetical changes. This helps beginners see exactly how these PHP functions behave in practice.