Operator precedence and evaluation in PHP - Mini Project: Build & Apply

Operator precedence and evaluation

📖 Scenario: Imagine you are calculating the total cost of items in a shopping cart with discounts and taxes applied. Understanding how PHP evaluates expressions with different operators helps you get the correct total.

🎯 Goal: You will write PHP code that uses variables and arithmetic operators. You will learn how operator precedence affects the calculation result.

📋 What You'll Learn

Create variables with exact values for item price, quantity, discount rate, and tax rate.

Create a variable for the discount threshold.

Calculate the total price applying discount only if quantity is above the threshold, then add tax.

Print the final total price.

💡 Why This Matters

🌍 Real World

Calculating prices with discounts and taxes is common in online shopping carts and billing systems.

💼 Career

Understanding operator precedence helps developers avoid bugs in financial calculations and write clear, correct code.

1

DATA SETUP: Create variables for item price, quantity, discount rate, and tax rate

Create four variables with these exact names and values: $itemPrice = 50;, $quantity = 3;, $discountRate = 0.1;, and $taxRate = 0.07;

PHP

<?php

// Create variables for item price, quantity, discount rate, and tax rate
// Your code here

Hint

Use the $ sign for variable names and assign the exact values given.

2

CONFIGURATION: Create a discount threshold variable

Create a variable called $discountThreshold and set it to 2

PHP

<?php
$itemPrice = 50;
$quantity = 3;
$discountRate = 0.1;
$taxRate = 0.07;
// Create a discount threshold variable
// Your code here

Hint

Remember to use $discountThreshold as the variable name and assign the value 2.

3

CORE LOGIC: Calculate total price with discount and tax using operator precedence

Create a variable called $totalPrice that calculates the total price as follows: multiply $itemPrice by $quantity, then subtract the discount only if $quantity is greater than $discountThreshold (discount is $itemPrice * $quantity * $discountRate), then add tax on the discounted amount (tax is $totalPrice * $taxRate). Use parentheses to ensure correct operator precedence.

PHP

<?php
$itemPrice = 50;
$quantity = 3;
$discountRate = 0.1;
$taxRate = 0.07;
$discountThreshold = 2;
// Calculate total price with discount and tax
// Your code here

Hint

Use the ternary operator condition ? true_value : false_value to apply discount only if quantity is above threshold. Use parentheses to control the order of operations.

4

OUTPUT: Print the final total price

Write a print statement to display the value of $totalPrice.

PHP

 $discountThreshold) ? $itemPrice * $quantity * $discountRate : 0)) * (1 + $taxRate);
// Print the total price
// Your code here

Hint

Use print($totalPrice); to show the final result.

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