Operator precedence and evaluation in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP code with mixed operators?

Consider the following PHP code snippet. What will it output?

PHP

<?php
$result = 3 + 4 * 2 / (1 - 5) ** 2 ** 3;
echo $result;
?>

A 7

B 11

C 3.0001220703125

D Syntax Error

💡 Hint

Remember that exponentiation (**) is right-associative and has higher precedence than multiplication and division.

The expression evaluates the exponentiation first, then multiplication and division, and finally addition. The result is approximately 3.0001220703125.

❓ Predict Output

intermediate

What is the output of this PHP code with logical and comparison operators?

What will this PHP code print?

PHP

<?php
$a = true;
$b = false;
$c = false;
$result = $a && $b || $c;
echo (int)$result;
?>

A 1

B 0

C Syntax Error

D null

💡 Hint

Logical AND (&&) has higher precedence than logical OR (||).

The expression is evaluated as ($a && $b) || $c, which is (true && false) || false = false || false = false, so output is 0.

🔧 Debug

advanced

What error does this PHP code raise due to operator precedence?

Examine the following PHP code. What error will it produce when run?

PHP

<?php
$value = 5;
if ($value > 3 && $value < 10 || $value = 0) {
    echo 'Valid';
} else {
    echo 'Invalid';
}
?>

A Warning: Assignment inside condition without parentheses

B No error, outputs 'Valid'

C Parse error: syntax error

D Outputs 'Invalid'

💡 Hint

Assignment (=) inside conditions can cause warnings if not grouped properly.

The assignment inside the condition is allowed but triggers a warning because it is ambiguous without parentheses. The expression evaluates with assignment having lower precedence than logical operators.

❓ Predict Output

advanced

What is the output of this PHP code with ternary and concatenation?

What will this PHP code print?

PHP

<?php
$score = 85;
$message = 'Result: ' . $score > 80 ? 'Pass' : 'Fail';
echo $message;
?>

A Pass

B Result: Pass

C Fail

D Result: 1

💡 Hint

Concatenation (.) has lower precedence than comparison (>) and ternary (?:).

The expression is parsed as ('Result: ' . $score) > 80 ? 'Pass' : 'Fail'. The concatenation results in a string, which when compared to 80 converts to 0, so 0 > 80 is false, so 'Fail' is chosen. But echo prints the boolean result of the comparison concatenation, which is 1 (true) or empty string (false). Actually, operator precedence causes unexpected behavior here, resulting in 'Result: 1'.

❓ Predict Output

expert

What is the value of $x after this PHP code runs?

Analyze this PHP code and determine the final value of $x.

PHP

<?php
$x = 2;
$x *= $x + 3;
echo $x;
?>

A 10

B 25

C 7

D Syntax Error

💡 Hint

Remember that the right side of the *= operator is evaluated before multiplication.

The expression is evaluated as $x = $x * ($x + 3) = 2 * (2 + 3) = 2 * 5 = 10.

Practice

1. In PHP, which operator has the highest precedence among these: +, *, -, /?

easy

A. All have the same precedence

B. Addition (+)

C. Subtraction (-)

D. Multiplication (*) and division (/)

Solution

1. Step 1: Recall operator precedence rules

   Multiplication (*) and division (/) have higher precedence than addition (+) and subtraction (-).

2. Step 2: Compare given operators

   Among +, -, *, /, the * and / operators are evaluated first.

3. Final Answer:

   Multiplication (*) and division (/) -> Option D

4. Quick Check:

   Higher precedence = * and / [OK]

Hint: Remember: multiply/divide before add/subtract [OK]

Common Mistakes:

• Thinking addition has higher precedence
• Assuming all operators have equal precedence
• Confusing subtraction with division precedence

2. Which of the following PHP expressions is syntactically correct and respects operator precedence?

easy

A. $result = 5 + 3 * 2;

B. $result = 5 + * 3 2;

C. $result = 5 + (3 * 2;

D. $result = 5 + 3 *;

Solution

1. Step 1: Check syntax of each expression

   $result = 5 + 3 * 2; is a valid PHP expression with correct operator placement. Options B, C, and D have syntax errors like misplaced operators or missing parentheses.

2. Step 2: Confirm operator precedence in $result = 5 + 3 * 2;

   Multiplication (*) happens before addition (+), so expression is valid and clear.

3. Final Answer:

   $result = 5 + 3 * 2; -> Option A

4. Quick Check:

   Valid syntax and precedence = $result = 5 + 3 * 2; [OK]

Hint: Check for misplaced operators and balanced parentheses [OK]

Common Mistakes:

• Leaving out parentheses
• Placing operators without operands
• Unbalanced parentheses

3. What is the output of the following PHP code?

$a = 4 + 2 * 3 - 1;

medium

A. 11

B. 17

C. 9

D. 15

Solution

1. Step 1: Apply operator precedence

   Multiplication (*) happens before addition (+) and subtraction (-). So calculate 2 * 3 = 6 first.

2. Step 2: Calculate the full expression

   Expression becomes 4 + 6 - 1 = 9. Then 4 + 6 = 10, 10 - 1 = 9.

3. Final Answer:

   9 -> Option A

4. Quick Check:

   Multiplication first, then add/subtract = 9 [OK]

Hint: Multiply before adding or subtracting [OK]

Common Mistakes:

• Adding before multiplying
• Subtracting before multiplying
• Ignoring operator precedence

4. Find the error in this PHP expression:

$result = 10 - 4 / 2 *;

medium

A. Division operator used incorrectly

B. Missing operand after the last operator

C. Subtraction operator has wrong precedence

D. Expression is correct

Solution

1. Step 1: Analyze the expression syntax

   The expression ends with an operator '*', but no operand follows it. This is a syntax error.

2. Step 2: Confirm the error type

   Operators must have operands on both sides. Missing operand after '*' causes error.

3. Final Answer:

   Missing operand after the last operator -> Option B

4. Quick Check:

   Operator must have operands both sides [OK]

Hint: Check if every operator has two operands [OK]

Common Mistakes:

• Ignoring missing operand errors
• Thinking operator precedence causes error
• Assuming expression is valid

5. Given the PHP expression:

$value = 8 + 2 * (5 - 3) / 2 - 1;

What is the correct value of $value after evaluation?

hard

A. 9

B. 10

C. 8

D. 11

Solution

1. Step 1: Evaluate parentheses first

   Calculate (5 - 3) = 2.

2. Step 2: Apply multiplication and division next

   Calculate 2 * 2 = 4, then 4 / 2 = 2.

3. Step 3: Perform addition and subtraction left to right

   Calculate 8 + 2 = 10, then 10 - 1 = 9.

4. Final Answer:

   8 -> Option C

5. Quick Check:

   Parentheses, then multiply/divide, then add/subtract = 8 [OK]

Hint: Calculate parentheses first, then multiply/divide, then add/subtract [OK]

Common Mistakes:

• Ignoring parentheses
• Doing addition before multiplication
• Dividing before multiplying inside parentheses