Operator precedence and evaluation in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is operator precedence in PHP?

Operator precedence determines the order in which different operators in an expression are evaluated. Operators with higher precedence are evaluated before those with lower precedence.

beginner

Which operator has higher precedence: multiplication (*) or addition (+)?

Multiplication (*) has higher precedence than addition (+), so it is evaluated first in an expression.

beginner

How does PHP evaluate the expression: 3 + 4 * 2?

PHP first multiplies 4 * 2 = 8, then adds 3 + 8 = 11. So the result is 11.

intermediate

What happens when operators have the same precedence?

When operators have the same precedence, PHP evaluates them based on their associativity, which can be left-to-right or right-to-left.

intermediate

Explain the difference between left-to-right and right-to-left associativity with an example.

Left-to-right associativity means operators are evaluated from left to right. For example, in 10 - 5 - 2, PHP evaluates (10 - 5) - 2 = 3. Right-to-left associativity means evaluation goes from right to left, like the assignment operator: $a = $b = 5 assigns 5 to $b first, then $a.

In PHP, which operator is evaluated first in the expression: 5 + 3 * 2?

A Addition (+)

B Multiplication (*)

C Both at the same time

D Depends on variable types

Multiplication (*) has higher precedence than addition (+), so 3 * 2 is evaluated first.

What is the result of the expression: 10 - 4 - 3 in PHP?

A 3

B 9

C 1

D Cannot be determined

Subtraction is left-associative, so PHP evaluates (10 - 4) - 3 = 6 - 3 = 3.

Which associativity does the assignment operator (=) have in PHP?

A Left-to-right

B No associativity

C Right-to-left

D Depends on context

The assignment operator (=) is right-to-left associative, so assignments happen from right to left.

In the expression: $a = 3 + 4 * 2, what is the value assigned to $a?

A 14

B 10

C 7

D 11

Multiplication happens first: 4 * 2 = 8, then addition: 3 + 8 = 11, so $a = 11.

If two operators have the same precedence and left-to-right associativity, how are they evaluated?

A From left to right

B Simultaneously

C Random order

D From right to left

Operators with left-to-right associativity are evaluated from left to right.

Describe how PHP decides the order of operations in an expression with multiple operators.

Think about which operators PHP looks at first and how it handles ties.

Explain the difference between operator precedence and associativity with simple examples.

Precedence is about which operator goes first; associativity is about direction when operators tie.

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