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.

Practice

1. Which of the following statements about isset in PHP is true?

easy

A. isset returns true even if the variable is null.

B. isset returns false if the variable is not set or is null.

C. isset checks if a variable is empty or zero.

D. isset returns true only if the variable is a string.

Solution

1. Step 1: Understand isset behavior

   isset returns false if the variable does not exist or is set to null.

2. Step 2: Analyze options

   isset returns false if the variable is not set or is null. correctly states this behavior: isset returns false if the variable is not set or is null. Options A, C, and D are incorrect because isset returns false for null, does not check emptiness or zero, and is not limited to strings.

3. Final Answer:

   isset returns false if the variable is not set or is null. -> Option B

4. Quick Check:

   isset false if null or unset [OK]

Hint: Remember: isset false if variable missing or null [OK]

Common Mistakes:

• Thinking isset returns true for null variables
• Confusing isset with empty
• Assuming isset checks variable type

2. Which of the following is the correct syntax to check if a variable $var is empty in PHP?

easy

A. if (empty($var)) { ... }

B. if (empty->$var) { ... }

C. if (is_empty($var)) { ... }

D. if (empty $var) { ... }

Solution

1. Step 1: Recall empty syntax

   The empty function requires parentheses around the variable: empty($var).

2. Step 2: Check each option

   if (empty($var)) { ... } uses correct syntax. if (empty $var) { ... } misses parentheses, C uses a non-existent function, and B uses invalid syntax.

3. Final Answer:

   if (empty($var)) { ... } -> Option A

4. Quick Check:

   empty requires parentheses [OK]

Hint: Use parentheses with empty: empty($var) [OK]

Common Mistakes:

• Omitting parentheses with empty
• Using is_empty instead of empty
• Using arrow operator incorrectly

3. What will be the output of the following PHP code?

$a = 0;
var_dump(isset($a));
var_dump(empty($a));
var_dump(is_null($a));

medium

A. bool(true)bool(false)bool(false)

B. bool(false)bool(true)bool(true)

C. bool(false)bool(false)bool(true)

D. bool(true)bool(true)bool(false)

Solution

1. Step 1: Evaluate isset($a) when $a = 0

   $a is set and not null, so isset($a) returns true.

2. Step 2: Evaluate empty($a) when $a = 0

   Zero is considered empty in PHP, so empty($a) returns true.

3. Step 3: Evaluate is_null($a) when $a = 0

   $a is integer zero, not null, so is_null($a) returns false.

4. Final Answer:

   bool(true)
   bool(true)
   bool(false) -> Option D

5. Quick Check:

   isset true, empty true for 0, is_null false [OK]

Hint: 0 is set and empty, but not null [OK]

Common Mistakes:

• Thinking isset returns false for 0
• Confusing empty with is_null
• Assuming 0 is null

4. Consider the following PHP code snippet:

$var = null;
if (empty($var)) {
    echo 'Empty';
} else {
    echo 'Not Empty';
}

Why does this code print "Empty"?

medium

A. Because empty returns false for null values.

B. Because empty only checks if variable is set.

C. Because empty returns true for null values.

D. Because empty throws an error for null values.

Solution

1. Step 1: Understand empty behavior with null

   empty returns true if the variable is not set or its value is considered empty, including null.

2. Step 2: Analyze the code output

   Since $var is null, empty($var) returns true, so "Empty" is printed.

3. Final Answer:

   Because empty returns true for null values. -> Option C

4. Quick Check:

   empty true for null [OK]

Hint: empty true for null or unset variables [OK]

Common Mistakes:

• Thinking empty only checks if variable is set
• Assuming empty returns false for null
• Expecting error on null with empty

5. You want to check if a variable $x exists and is not null, but also want to treat the value 0 as valid (not empty). Which combination of PHP functions correctly achieves this?

hard

A. if (isset($x) && $x !== null) { ... }

B. if (isset($x) && !empty($x)) { ... }

C. if (!is_null($x) && !empty($x)) { ... }

D. if (!empty($x) || isset($x)) { ... }

Solution

1. Step 1: Understand the requirement

   We want to check if $x exists and is not null, but 0 should be valid (not treated as empty).

2. Step 2: Analyze each option

   if (isset($x) && !empty($x)) { ... } fails because empty(0) is true, so 0 would be excluded. if (isset($x) && $x !== null) { ... } checks if $x is set and not null, allowing 0. if (!is_null($x) && !empty($x)) { ... } excludes 0 because of empty($x). if (!empty($x) || isset($x)) { ... } uses OR, which can be true even if $x is empty or null.

3. Final Answer:

   if (isset($x) && $x !== null) { ... } -> Option A

4. Quick Check:

   Use isset and strict null check to allow 0 [OK]

Hint: Use isset and !== null to allow zero values [OK]

Common Mistakes:

• Using empty excludes zero values
• Using OR instead of AND
• Confusing is_null with isset