Boolean type behavior in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Boolean evaluation of different values

What is the output of this PHP code snippet?

PHP

<?php
$values = [0, 1, -1, "0", "false", "", null, [], [0]];
foreach ($values as $v) {
    echo (bool)$v ? 'true ' : 'false ';
}
?>

A false true true true false false false false true

B false true false false false false false false true

C false true true false false false false false true

D false true true false true false false false true

💡 Hint

Remember how PHP treats strings and arrays when cast to boolean.

In PHP, the following values are considered false when cast to boolean: 0, "0", empty string "", null, and empty array []. All other values are true.

So the outputs correspond to these evaluations.

❓ Predict Output

intermediate

Boolean comparison with loose equality

What is the output of this PHP code?

PHP

<?php
var_dump(false == 0);
var_dump(false == '');
var_dump(false == null);
var_dump(false == 'false');
?>

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

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

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

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

💡 Hint

Loose equality in PHP converts types before comparing.

In PHP, false == 0, false == '', and false == null all evaluate to true because of type juggling. However, false == 'false' is false because 'false' is a non-empty string and converts to true in boolean context.

🔧 Debug

advanced

Unexpected boolean result in conditional

Why does this PHP code always print "No" even when $value is 1?

PHP

<?php
$value = 1;
if ($value == true) {
    echo "Yes";
} else {
    echo "No";
}
?>

A Because the code is correct and it should print "Yes", so the problem is elsewhere

B Because the variable $value is an integer and cannot be compared to boolean

C Because the code uses == instead of ===, causing unexpected type coercion

D Because == compares values loosely and 1 is not equal to true

💡 Hint

Test the code by running it and see what it prints.

The code actually prints "Yes" because 1 == true evaluates to true in PHP. So the problem is not in this snippet.

📝 Syntax

advanced

Boolean expression syntax error

Which option will cause a syntax error in PHP?

A if ($a && $b) { echo 'Yes'; }

B if ($a &&) { echo 'Yes'; }

C if ($a & $b) { echo 'Yes'; }

D if ($a and $b) { echo 'Yes'; }

💡 Hint

Look for incomplete boolean expressions.

Option B has a syntax error because the && operator is missing the right operand.

🚀 Application

expert

Count how many values are true in mixed array

Given this PHP array, how many elements evaluate to true when cast to boolean?

PHP

<?php
$items = [0, "", "0", null, false, true, 1, -1, [], [0], "false"];
$count = 0;
foreach ($items as $item) {
    if ((bool)$item) {
        $count++;
    }
}
echo $count;
?>

A 8

B 7

C 5

D 6

💡 Hint

Recall which values PHP treats as false.

False values are: 0, "", "0", null, false, and empty array []. All others are true.

Counting true values: true, 1, -1, [0], "false" → 5

Practice

1. In PHP, which of the following values is considered true when converted to a boolean?

easy

A. The string "false"

B. The integer 0

C. An empty string ""

D. Any non-zero number

Solution

1. Step 1: Understand PHP boolean conversion rules

   In PHP, zero (0), empty string (""), and the string "0" are considered false. Any non-zero number is true.

2. Step 2: Evaluate each option

   Any non-zero number is a non-zero number, so it converts to true. Options B and C are false values. The string "false" is a non-empty string, so it is true, but "false" as a string is not false boolean.

3. Final Answer:

   Any non-zero number -> Option D

4. Quick Check:

   Non-zero number = true [OK]

Hint: Non-zero numbers are true; zero and empty strings are false [OK]

Common Mistakes:

• Thinking string "false" is boolean false
• Assuming zero is true
• Confusing empty string with true

2. Which of the following is the correct way to explicitly cast a variable $var to boolean in PHP?

easy

A. $bool = boolean($var);

B. $bool = bool($var);

C. $bool = (bool) $var;

D. $bool = to_bool($var);

Solution

1. Step 1: Recall PHP casting syntax

   PHP uses (bool) or (boolean) to cast variables to boolean type. The function bool() or boolean() does not exist.

2. Step 2: Check each option

   $bool = (bool) $var; uses (bool) casting which is correct. Options B, C, and D use invalid function calls.

3. Final Answer:

   $bool = (bool) $var; -> Option C

4. Quick Check:

   Use (bool) for casting [OK]

Hint: Use (bool) before variable to cast to boolean [OK]

Common Mistakes:

• Using non-existent functions like bool()
• Confusing casting with function calls
• Using wrong keywords like boolean()

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

$a = 0;
$b = "0";
$c = "false";
$d = [];

echo (int)(bool)$a . "," . (int)(bool)$b . "," . (int)(bool)$c . "," . (int)(bool)$d;

medium

A. 0,0,1,0

B. 0,1,1,0

C. 0,0,0,0

D. 1,1,1,1

Solution

1. Step 1: Convert each variable to boolean

   $a = 0 is false, $b = "0" is false, $c = "false" is a non-empty string so true, $d = [] (empty array) is false.

2. Step 2: Cast booleans to int and concatenate

   False becomes 0, true becomes 1. So output is "0,0,1,0".

3. Final Answer:

   0,0,1,0 -> Option A

4. Quick Check:

   False=0, True=1 [OK]

Hint: Empty, zero, and "0" are false; non-empty strings true [OK]

Common Mistakes:

• Assuming string "false" is false boolean
• Thinking empty array is true
• Mixing string and boolean conversion

4. Consider this PHP code snippet:

$value = "";
if ($value == true) {
    echo "Yes";
} else {
    echo "No";
}

Why does it output "No" and how to fix it to check if $value is truthy?

medium

A. Because empty string is false; use (bool)$value in if condition

B. Because == does loose comparison; use === true to fix

C. Because $value is string; cast to int before comparison

D. Because PHP treats all strings as false; use strlen() instead

Solution

1. Step 1: Understand why empty string is false

   In PHP, empty string "" is considered false in boolean context, so if ($value == true) fails.

2. Step 2: Fix by casting to boolean in condition

   Use if ((bool)$value) or simply if ($value) to check truthiness correctly.

3. Final Answer:

   Because empty string is false; use (bool)$value in if condition -> Option A

4. Quick Check:

   Empty string = false; cast to bool to check [OK]

Hint: Empty string is false; cast variable to bool in conditions [OK]

Common Mistakes:

• Using == true instead of casting
• Assuming all strings are true
• Trying to cast string to int unnecessarily

5. You want to filter an array in PHP to keep only truthy values (non-empty, non-zero, etc.). Which code correctly uses boolean behavior to achieve this?

$arr = [0, 1, "", "hello", false, true, null, [], [1]];
$filtered = array_filter($arr, function($v) {
    // What to put here?
});
print_r($filtered);

hard

A. return $v == true;

B. return (bool)$v;

C. return $v === true;

D. return !empty($v);

Solution

1. Step 1: Understand array_filter callback

   The callback should return true for values to keep. Casting to (bool) returns true for all truthy values.

2. Step 2: Compare options

   return (bool)$v; casts to boolean correctly. return $v == true; uses loose comparison which may include unwanted values. return $v === true; keeps only true boolean, excluding truthy values. return !empty($v); excludes some truthy values like true boolean but empty arrays are false.

3. Final Answer:

   return (bool)$v; -> Option B

4. Quick Check:

   Cast to bool to filter truthy values [OK]

Hint: Use (bool) cast in filter callback to keep truthy values [OK]

Common Mistakes:

• Using strict === true excludes truthy but non-true values
• Using == true includes unwanted falsy values
• Using empty() excludes some truthy values