Logical operators in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the logical AND operator (&&) do in PHP?

It returns true only if both conditions on its sides are true. Otherwise, it returns false.

beginner

What is the result of the expression: true || false?

The result is true because the logical OR operator (||) returns true if at least one condition is true.

beginner

Explain the logical NOT operator (!) in PHP.

The NOT operator (!) reverses the boolean value. If the value is true, it becomes false; if false, it becomes true.

intermediate

What is the difference between 'and' and '&&' in PHP?

'&&' has higher precedence than 'and'. This means '&&' is evaluated before other operators, while 'and' is evaluated later, which can affect the result.

beginner

How do logical operators help in decision making in PHP?

They combine multiple conditions to decide if a block of code should run, making programs smarter and able to handle complex choices.

Which operator returns true only if both conditions are true?

A &&

B ||

C !

D xor

The && operator is the logical AND, which returns true only if both sides are true.

What does the expression (!false) evaluate to?

A null

B false

C true

D error

The ! operator reverses false to true.

Which operator has higher precedence in PHP?

A and

B &&

C or

D xor

The && operator has higher precedence than 'and'.

What is the result of true || false?

A true

B false

C null

D error

The || operator returns true if at least one condition is true.

Which operator returns true if exactly one condition is true?

A &&

B ||

C !

D xor

The xor operator returns true only if one condition is true, but not both.

Explain how logical AND (&&) and OR (||) operators work in PHP with simple examples.

Think about combining two yes/no questions.

Describe the difference between 'and' and '&&' operators in PHP and why it matters.

Consider how PHP reads expressions with these operators.

Practice

1. Which logical operator in PHP returns true only if both conditions are true?

easy

A. && (AND)

B. || (OR)

C. ! (NOT)

D. xor

Solution

1. Step 1: Understand the AND operator

   The AND operator && returns true only if both conditions it connects are true.

2. Step 2: Compare with other operators

   The OR operator || returns true if at least one condition is true, NOT ! negates a condition, and xor returns true if exactly one condition is true.

3. Final Answer:

   && (AND) -> Option A

4. Quick Check:

   AND = both true [OK]

Hint: AND means both must be true to get true [OK]

Common Mistakes:

• Confusing OR (||) with AND (&&)
• Thinking NOT (!) combines two conditions
• Assuming xor behaves like AND

2. Which of the following is the correct syntax for the logical OR operator in PHP?

easy

A. !

B. &&

C. ||

D. and

Solution

1. Step 1: Identify the OR operator syntax

   In PHP, the logical OR operator is written as ||.

2. Step 2: Differentiate from other operators

   && is AND, ! is NOT, and and is a lower precedence alternative but not the common OR operator symbol.

3. Final Answer:

   || -> Option C

4. Quick Check:

   OR = || symbol [OK]

Hint: OR operator is double vertical bars || [OK]

Common Mistakes:

• Using single | instead of ||
• Confusing AND (&&) with OR (||)
• Using 'and' keyword instead of ||

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

$a = true;
$b = false;
if ($a && !$b) {
    echo 'Yes';
} else {
    echo 'No';
}

medium

A. No

B. Yes

C. Syntax Error

D. Nothing

Solution

1. Step 1: Evaluate the NOT operator on $b

   $b is false, so !$b becomes true.

2. Step 2: Evaluate the AND condition

   $a is true and !$b is true, so $a && !$b is true.

3. Final Answer:

   Yes -> Option B

4. Quick Check:

   true AND true = true [OK]

Hint: NOT flips false to true, AND needs both true [OK]

Common Mistakes:

• Ignoring the NOT operator effect
• Assuming false && true is true
• Confusing output strings

4. Find the error in this PHP code snippet:

$x = true;
$y = false;
if ($x || ! $y) {
    echo 'True';
 else {
    echo 'False';
}

medium

A. Missing closing brace for if block

B. Missing semicolon after echo statement

C. Incorrect use of logical OR operator

D. Missing closing parenthesis in if condition

Solution

1. Step 1: Check the if statement syntax

   The if condition has correct parentheses and logical operators.

2. Step 2: Check the braces for if-else blocks

   The if block opens with { but does not have a closing } before the else block starts.

3. Final Answer:

   Missing closing brace for if block -> Option A

4. Quick Check:

   Braces must match for if-else [OK]

Hint: Count opening and closing braces carefully [OK]

Common Mistakes:

• Assuming missing parenthesis causes error
• Thinking logical OR syntax is wrong
• Ignoring missing brace errors

5. Given the variables:

$a = false;
$b = true;
$c = false;

Which expression evaluates to true?
($a || $b) && !$c

hard

A. null

B. false

C. Syntax Error

D. true

Solution

1. Step 1: Evaluate the OR inside parentheses

   $a is false, $b is true, so $a || $b is true.

2. Step 2: Evaluate the NOT operator on $c

   $c is false, so !$c is true.

3. Step 3: Evaluate the AND between results

   true AND true equals true.

4. Final Answer:

   true -> Option D

5. Quick Check:

   (false OR true) AND NOT false = true [OK]

Hint: Evaluate inside parentheses first, then NOT, then AND [OK]

Common Mistakes:

• Ignoring operator precedence
• Misapplying NOT operator
• Assuming OR returns false if one operand is false