Operator precedence and evaluation in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add 2 and 3, then multiply the result by 4.

PHP

<?php
$result = (2 + 3) [1] 4;
echo $result;
?>

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

A*

B/

C-

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using + instead of * causes the result to be 5 + 4 = 9, which is incorrect.

Forgetting parentheses changes the order of operations.

The multiplication operator (*) has higher precedence than addition, but parentheses force addition first. So (2 + 3) * 4 equals 20.

2fill in blank

medium

Complete the code to check if 5 is greater than 3 and less than 10.

PHP

<?php
if (5 [1] 3 && 5 < 10) {
    echo 'True';
} else {
    echo 'False';
}
?>

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

A!=

B>

C==

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' reverses the comparison and gives false.

Using '==' checks equality, not greater than.

The expression 5 > 3 checks if 5 is greater than 3, which is true. Combined with 5 < 10, the whole condition is true.

3fill in blank

hard

Fix the error in the expression to correctly calculate 10 minus 3 times 2.

PHP

<?php
$result = 10 - 3 [1] 2;
echo $result;
?>

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

A+

B/

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '*' changes the result incorrectly.

Using '-' again causes wrong calculation.

Multiplication (*) has higher precedence than subtraction (-), so 3 * 2 is calculated first, then subtracted from 10, resulting in 4.

4fill in blank

hard

Fill both blanks to create an array with keys as numbers and values as their squares, but only for numbers less than 5.

PHP

<?php
$squares = [];
for ($i = 1; $i <= 6; $i++) if ($i < 5) $squares[[1]] = [2];
print_r($squares);
?>

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

A$i

B$i * $i

C$i + $i

D$i - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of multiplication for squares.

Using wrong keys that don't match the numbers.

The key is the number $i, and the value is its square $i * $i. The condition limits to numbers less than 5.

5fill in blank

hard

Fill all three blanks to create an associative array with uppercase keys, values as numbers, and only include values greater than 2.

PHP

<?php
$numbers = [1, 2, 3, 4];
$result = [];
foreach ($numbers as [3]) if ([2] > 2) $result[[1]] = [2];
print_r($result);
?>

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

Astrtoupper($num)

B$num

D$number

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

Not converting keys to uppercase.

The key is the uppercase string of the number (strtoupper($num)), the value is the number itself ($num), and the loop variable is $num. The condition filters values greater than 2.

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