Custom exception classes in PHP - Mini Project: Build & Apply

Custom Exception Classes in PHP

📖 Scenario: Imagine you are building a simple banking system. You want to handle errors like withdrawing too much money or depositing invalid amounts in a clear way.

🎯 Goal: You will create custom exception classes to handle specific banking errors and use them in your code to show meaningful error messages.

📋 What You'll Learn

Create a custom exception class called InsufficientFundsException

Create a custom exception class called InvalidDepositException

Write a function withdraw that throws InsufficientFundsException if withdrawal amount is more than balance

Write a function deposit that throws InvalidDepositException if deposit amount is less than or equal to zero

Use try and catch blocks to handle these exceptions and print error messages

💡 Why This Matters

🌍 Real World

Custom exceptions help you handle specific errors in your programs clearly, like banking errors or user input mistakes.

💼 Career

Understanding custom exceptions is important for writing robust, maintainable code in professional software development.

1

Create the initial balance variable

Create a variable called $balance and set it to 1000.

PHP

<?php

// Create a variable called $balance and set it to 1000
// Your code here

Hint

Use $balance = 1000; to set the starting balance.

2

Create custom exception classes

Create two custom exception classes called InsufficientFundsException and InvalidDepositException that both extend the built-in Exception class.

PHP

<?php

$balance = 1000;

// Create class InsufficientFundsException that extends Exception
// Create class InvalidDepositException that extends Exception
// Your code here

Hint

Use class ClassName extends Exception {} to create custom exceptions.

3

Write withdraw and deposit functions with exceptions

Write a function called withdraw that takes $amount and throws InsufficientFundsException if $amount is greater than $balance. Write a function called deposit that takes $amount and throws InvalidDepositException if $amount is less than or equal to zero. Both functions should update the global $balance variable accordingly.

PHP

 balance
// Write function deposit that throws InvalidDepositException if amount <= 0
// Your code here

Hint

Use throw new ExceptionName("message") inside the functions to raise errors.

4

Use try-catch blocks to handle exceptions and print results

Use try and catch blocks to call withdraw(1500) and deposit(-100). Catch InsufficientFundsException and InvalidDepositException separately and print their messages. Then print the final $balance.

PHP

 $balance) {
        throw new InsufficientFundsException("Cannot withdraw $amount: insufficient funds.");
    }
    $balance -= $amount;
}

function deposit($amount) {
    global $balance;
    if ($amount <= 0) {
        throw new InvalidDepositException("Cannot deposit $amount: invalid amount.");
    }
    $balance += $amount;
}

// Use try-catch to call withdraw(1500) and deposit(-100)
// Catch exceptions and print messages
// Print final balance
// Your code here

Hint

Use separate try blocks for each function call and catch the specific exceptions to print their messages.

Practice

1.

What is the main purpose of creating a custom exception class in PHP?

easy

A. To define specific error types for clearer error handling

B. To replace the built-in Exception class completely

C. To speed up the execution of the program

D. To automatically fix errors in the code

Solution

1. Step 1: Understand the role of custom exceptions

   Custom exceptions allow programmers to create specific error types that represent particular problems in the code.

2. Step 2: Compare with other options

   Custom exceptions do not replace the built-in Exception class, nor do they speed up execution or fix errors automatically.

3. Final Answer:

   To define specific error types for clearer error handling -> Option A

4. Quick Check:

   Custom exceptions = Specific error types [OK]

Hint: Custom exceptions clarify errors by type, not fix or speed [OK]

Common Mistakes:

• Thinking custom exceptions replace built-in Exception
• Believing custom exceptions fix errors automatically
• Assuming custom exceptions improve performance

2.

Which of the following is the correct way to declare a custom exception class named MyError in PHP?

?

easy

A. class MyError extends Error {}

B. class MyError implements Exception {}

C. class MyError extends Exception {}

D. class MyError inherits Exception {}

Solution

1. Step 1: Recall PHP syntax for custom exceptions

   Custom exceptions extend the built-in Exception class using the keyword extends.

2. Step 2: Check each option

   class MyError extends Exception {} correctly uses extends Exception. class MyError implements Exception {} wrongly uses implements, which is for interfaces. class MyError extends Error {} extends Error, which is a different base class. class MyError inherits Exception {} uses invalid syntax inherits.

3. Final Answer:

   class MyError extends Exception {} -> Option C

4. Quick Check:

   Custom exception syntax = extends Exception [OK]

Hint: Use 'extends Exception' to create custom exceptions [OK]

Common Mistakes:

• Using 'implements' instead of 'extends'
• Using 'inherits' keyword which is invalid
• Extending Error instead of Exception

3.

What will be the output of the following PHP code?

class MyException extends Exception {}

try {
    throw new MyException("Custom error happened");
} catch (MyException $e) {
    echo $e->getMessage();
} catch (Exception $e) {
    echo "General error";
}

medium

A. Custom error happened

B. General error

C. Fatal error: Uncaught exception

D. No output

Solution

1. Step 1: Understand the thrown exception type

   The code throws an exception of type MyException.

2. Step 2: Check catch blocks order and matching

   The first catch block matches MyException, so it catches the exception and echoes the message.

3. Final Answer:

   Custom error happened -> Option A

4. Quick Check:

   Matching catch block runs = Custom message [OK]

Hint: Catch specific exceptions before general ones [OK]

Common Mistakes:

• Expecting general catch to run first
• Thinking exception is uncaught
• Ignoring exception message

4.

Identify the error in this PHP code that defines and uses a custom exception:

class MyException extends Exception {}

try {
    throw new MyException("Error!");
} catch (Exception $e) {
    echo $e->getMessage();
} catch (MyException $e) {
    echo "Caught MyException";
}

medium

A. No error; code runs fine

B. The catch blocks are in wrong order; specific should come before general

C. The throw statement syntax is incorrect

D. The class should implement Exception interface

Solution

1. Step 1: Analyze catch block order

   The first catch block catches all Exception types, including MyException, so the second catch block is unreachable.

2. Step 2: Correct catch order

   Specific exceptions like MyException must be caught before the general Exception to avoid unreachable code.

3. Final Answer:

   The catch blocks are in wrong order; specific should come before general -> Option B

4. Quick Check:

   Specific catch before general catch [OK]

Hint: Catch specific exceptions before general ones to avoid unreachable code [OK]

Common Mistakes:

• Placing general catch before specific catch
• Thinking implements is needed for exceptions
• Misunderstanding throw syntax

5.

Given this custom exception class and code, what will be the output?

class ValidationException extends Exception {
    public function getCustomMessage() {
        return "Validation failed: " . $this->getMessage();
    }
}

try {
    throw new ValidationException("Invalid input");
} catch (ValidationException $e) {
    echo $e->getCustomMessage();
} catch (Exception $e) {
    echo "General error";
}

hard

A. Fatal error: Call to undefined method

B. Invalid input

C. General error

D. Validation failed: Invalid input

Solution

1. Step 1: Understand the custom method in exception

   The class ValidationException adds a method getCustomMessage() that returns a formatted message.

2. Step 2: Check which catch block runs and output

   The thrown exception is ValidationException, so the first catch block runs and echoes the custom message.

3. Final Answer:

   Validation failed: Invalid input -> Option D

4. Quick Check:

   Custom method called on caught exception [OK]

Hint: Call custom methods on caught custom exceptions [OK]

Common Mistakes:

• Expecting default getMessage() output only
• Thinking general catch block runs first
• Assuming method does not exist