Isset, empty, and is_null behavior in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the variable $var is set.

PHP

<?php
$var = 5;
if ([1]($var)) {
    echo "Variable is set.";
} else {
    echo "Variable is not set.";
}
?>

Drag options to blanks, or click blank then click option'

Adefined

Bempty

Cisset

Dis_null

Attempts:

3 left

💡 Hint

Common Mistakes

Using empty() instead of isset() which also checks for empty values.

Using is_null() which only checks if a variable is null, but does not check if it is set.

The isset() function checks if a variable is set and is not null.

2fill in blank

medium

Complete the code to check if the variable $var is empty.

PHP

<?php
$var = 0;
if ([1]($var)) {
    echo "Variable is empty.";
} else {
    echo "Variable is not empty.";
}
?>

Drag options to blanks, or click blank then click option'

Astrlen

Bisset

Cis_null

Dempty

Attempts:

3 left

💡 Hint

Common Mistakes

Using isset() which returns true even if the variable is 0 or empty string.

Using is_null() which only checks if the variable is null.

The empty() function checks if a variable is empty. It returns true for values like 0, '', null, false, and empty arrays.

3fill in blank

hard

Fix the error in the code to correctly check if $var is null.

PHP

<?php
$var = null;
if ([1]($var)) {
    echo "Variable is null.";
} else {
    echo "Variable is not null.";
}
?>

Drag options to blanks, or click blank then click option'

Ais_null

Bempty

Cisset

Ddefined

Attempts:

3 left

💡 Hint

Common Mistakes

Using isset() which returns false if variable is null but does not check for null explicitly.

Using empty() which returns true for many values, not just null.

The is_null() function checks if a variable is exactly null.

4fill in blank

hard

Fill both blanks to include only words with length greater than 3 in the associative array.

PHP

<?php
$words = ['apple', 'cat', 'dog', 'banana'];
$result = [];
foreach ($words as $word) {
    if ([1]($word) [2] 3) {
        $result[$word] = strlen($word);
    }
}
print_r($result);
?>

Drag options to blanks, or click blank then click option'

Astrlen

B<

C>

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using count() which works for arrays, not strings.

Using '<' instead of '>' which would select shorter words.

Use strlen() to get the length of the word and check if it is greater than 3.

5fill in blank

hard

Fill all three blanks to create an array filtering code that keeps only non-empty, set, and non-null values.

PHP

<?php
$values = [0, null, '', 'hello', false, 42];
$result = [];
foreach ($values as $val) {
    if (![1]($val) && [2]($val) && ![3]($val)) {
        $result[] = $val;
    }
}
print_r($result);
?>

Drag options to blanks, or click blank then click option'

Aempty

Bisset

Cis_null

Dis_string

Attempts:

3 left

💡 Hint

Common Mistakes

Using is_null() directly in the condition without negation.

Using is_string() which is unrelated to emptiness or being set.

The code filters values that are not empty and are set. The empty() function checks for empty values, isset() checks if variable is set, and is_null() can be used to check for null values if needed.

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