Boolean type behavior in PHP - Time & Space Complexity

Time Complexity: Boolean type behavior

O(n)

Understanding Time Complexity

Let's explore how the time it takes to work with Boolean values changes as we use more of them.

We want to see how the program's steps grow when handling many Booleans.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

$bools = array_fill(0, $n, true);
$result = true;
foreach ($bools as $b) {
    $result = $result && $b;
}
echo $result ? 'All true' : 'Not all true';
    

This code checks if all Boolean values in an array are true by going through each one.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through each Boolean in the array.
• How many times: Exactly once for each Boolean, so n times.

How Execution Grows With Input

As the number of Booleans increases, the steps to check them all grow in a straight line.

Input Size (n)	Approx. Operations
10	10 checks
100	100 checks
1000	1000 checks

Pattern observation: The work grows evenly with the number of Booleans.

Final Time Complexity

Time Complexity: O(n)

This means the time to check all Booleans grows directly with how many there are.

Common Mistake

[X] Wrong: "Checking Booleans is always instant, so time doesn't grow with more values."

[OK] Correct: Even simple checks take time when repeated many times, so more Booleans mean more steps.

Interview Connect

Understanding how simple operations add up helps you explain code efficiency clearly and confidently.

Self-Check

"What if we stopped checking as soon as we found a false value? How would the time complexity change?"

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