Isset, empty, and is_null behavior in PHP

Introduction

These functions help check if a variable exists, is empty, or is null. They make your code safer and avoid errors.

Before using a variable to make sure it exists.

To check if a form input is empty before processing it.

To find out if a variable has no value or is set to null.

When you want to avoid warnings from undefined variables.

To decide if you should assign a default value to a variable.

Syntax

PHP

isset(mixed $var): bool
empty(mixed $var): bool
is_null(mixed $var): bool

isset returns true if the variable exists and is not null.

empty returns true if the variable is empty (like 0, '0', '', null, false, or not set).

Examples

Variable $a is set to 5, so isset is true, empty is false, and is_null is false.

PHP

$a = 5;
isset($a); // true
empty($a); // false
is_null($a); // false

Variable $b is set to null, so isset is false, empty is true, and is_null is true.

PHP

$b = null;
isset($b); // false
empty($b); // true
is_null($b); // true

Variable $c is not set, so isset is false, empty is true, but is_null causes a warning if used.

PHP

unset($c);
isset($c); // false
empty($c); // true
// is_null($c) gives warning if $c not defined

Sample Program

This program shows how isset, empty, and is_null behave with different variable states: zero, null, and undefined.

PHP

<?php
$a = 0;
$b = null;
// $c is not set

echo "isset($a): " . (isset($a) ? 'true' : 'false') . "\n";
echo "empty($a): " . (empty($a) ? 'true' : 'false') . "\n";
echo "is_null($a): " . (is_null($a) ? 'true' : 'false') . "\n";

echo "isset($b): " . (isset($b) ? 'true' : 'false') . "\n";
echo "empty($b): " . (empty($b) ? 'true' : 'false') . "\n";
echo "is_null($b): " . (is_null($b) ? 'true' : 'false') . "\n";

echo "isset($c): " . (isset($c) ? 'true' : 'false') . "\n";
echo "empty($c): " . (empty($c) ? 'true' : 'false') . "\n";
// echo "is_null($c): " . (is_null($c) ? 'true' : 'false') . "\n"; // Warning if uncommented
?>

Important Notes

empty treats 0, '0', false, null, '', and undefined variables as empty.

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

Using is_null on an undefined variable causes a warning, so check with isset first.

Summary

isset checks if a variable exists and is not null.

empty checks if a variable is empty or not set.

is_null checks if a variable is exactly null.

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