Throwing exceptions in PHP - Mini Project: Build & Apply

Throwing exceptions

📖 Scenario: Imagine you are creating a simple bank account system. You want to make sure that if someone tries to withdraw more money than they have, the program will stop and show an error message.

🎯 Goal: You will build a PHP program that throws an exception when a withdrawal amount is greater than the account balance.

📋 What You'll Learn

Create a variable to hold the account balance

Create a variable for the withdrawal amount

Use an if statement to check if withdrawal is greater than balance

Throw an exception with a clear message if withdrawal is too large

Print a success message if withdrawal is allowed

💡 Why This Matters

🌍 Real World

Throwing exceptions is useful in real programs to stop execution when something goes wrong, like invalid input or errors.

💼 Career

Understanding exceptions is important for writing reliable and safe code in many programming jobs.

1

Set up account balance and withdrawal amount

Create a variable called $balance and set it to 1000. Create another variable called $withdrawal and set it to 1200.

PHP

<?php

// Create $balance and $withdrawal variables

Hint

Use simple assignment to create the variables with the exact names and values.

2

Prepare to check withdrawal amount

Create a variable called $canWithdraw and set it to true. This will help us decide if withdrawal is allowed.

PHP

<?php

$balance = 1000;
$withdrawal = 1200;

// Create $canWithdraw variable and set it to true

Hint

This variable will be used to track if withdrawal is possible.

3

Throw exception if withdrawal is too large

Use an if statement to check if $withdrawal is greater than $balance. Inside the if, throw a new Exception with the message 'Withdrawal amount exceeds balance.'.

PHP

<?php

$balance = 1000;
$withdrawal = 1200;
$canWithdraw = true;

// Add if statement to throw Exception if withdrawal is too large

Hint

Use throw new Exception('message'); inside the if block.

4

Print success message if no exception

Add a print statement that outputs 'Withdrawal successful.' after the if block.

PHP

 $balance) {
    throw new Exception('Withdrawal amount exceeds balance.');
}

// Print success message

Hint

The program will stop and show an error message because the withdrawal is too large.

Practice

1. What does the throw statement do in PHP?

easy

A. It catches an exception to handle an error.

B. It creates and signals an exception to indicate an error.

C. It defines a new function for error handling.

D. It stops the script without any error message.

Solution

1. Step 1: Understand the purpose of throw

   The throw statement is used to create and signal an exception when an error occurs.

2. Step 2: Differentiate from other error handling keywords

   catch handles exceptions, but throw actually creates and signals them.

3. Final Answer:

   It creates and signals an exception to indicate an error. -> Option B

4. Quick Check:

   throw = signal error [OK]

Hint: Remember: throw = create error, catch = handle error [OK]

Common Mistakes:

• Confusing throw with catch
• Thinking throw stops script silently
• Assuming throw defines functions

2. Which of the following is the correct syntax to throw an exception with the message "Error occurred" in PHP?

easy

A. throw new Exception("Error occurred");

B. throw Exception("Error occurred");

C. throw Exception new("Error occurred");

D. throw new Exception(Error occurred);

Solution

1. Step 1: Recall the syntax for throwing exceptions

   In PHP, you must use throw new Exception("message") to throw an exception with a message.

2. Step 2: Check each option for syntax correctness

   Only throw new Exception("Error occurred"); matches the correct syntax exactly. The other options either omit the new keyword, have the wrong order of keywords, or lack quotes around the message.

3. Final Answer:

   throw new Exception("Error occurred"); -> Option A

4. Quick Check:

   Correct syntax = throw new Exception(message) [OK]

Hint: Use 'throw new Exception("message")' exactly [OK]

Common Mistakes:

• Omitting the 'new' keyword
• Not quoting the message string
• Wrong order of keywords

3. What will be the output of the following PHP code?

try {
    throw new Exception("Oops!");
    echo "No error";
} catch (Exception $e) {
    echo $e->getMessage();
}

medium

A. No error

B. Fatal error

C. Exception thrown but no output

D. Oops!

Solution

1. Step 1: Analyze the try block

   The code throws an exception with message "Oops!" before the echo statement, so "No error" is never printed.

2. Step 2: Understand the catch block behavior

   The catch block catches the exception and echoes the message from the exception, which is "Oops!".

3. Final Answer:

   Oops! -> Option D

4. Quick Check:

   Exception message printed = Oops! [OK]

Hint: Exception stops try block; catch prints message [OK]

Common Mistakes:

• Expecting code after throw to run
• Thinking no output appears
• Confusing exception message with error

4. Identify the error in this PHP code snippet:

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

medium

A. Exception class is not imported.

B. Missing semicolon after throw statement.

C. Method message() does not exist; should use getMessage().

D. Catch block syntax is incorrect.

Solution

1. Step 1: Check the catch block method call

   The Exception class has a method getMessage() to get the error message, not message().

2. Step 2: Verify other syntax elements

   The semicolon after throw is present, Exception class is built-in, and catch syntax is correct.

3. Final Answer:

   Method message() does not exist; should use getMessage(). -> Option C

4. Quick Check:

   Use getMessage() to get exception text [OK]

Hint: Use getMessage() method to read exception message [OK]

Common Mistakes:

• Calling non-existent message() method
• Missing semicolons (not in this case)
• Incorrect catch syntax

5. You want to throw an exception only if a variable $age is less than 18. Which code correctly does this and catches the exception to print its message "Too young"?

hard

A. try {
if ($age < 18) {
throw new Exception("Too young");
}
} catch (Exception $e) {
echo $e->getMessage();
}

B. try {
if ($age < 18) {
throw new Exception("Too young");
}
} catch (Exception $e) {
echo "Too young";
}

C. try {
if ($age < 18) {
throw Exception("Too young");
}
} catch (Exception $e) {
echo $e->getMessage();
}

D. if ($age < 18) {
throw new Exception("Too young");
} catch (Exception $e) {
echo $e->getMessage();
}

Solution

1. Step 1: Identify correct try-catch structure

   The exception must be thrown inside a try block to be caught properly. try { if ($age < 18) { throw new Exception("Too young"); } } catch (Exception $e) { echo $e->getMessage(); } correctly wraps the throw inside try.

2. Step 2: Check exception throwing syntax and catch output

   try { if ($age < 18) { throw new Exception("Too young"); } } catch (Exception $e) { echo $e->getMessage(); } uses correct syntax throw new Exception("Too young") and prints the exception message with echo $e->getMessage();.

3. Final Answer:

   try { if ($age < 18) { throw new Exception("Too young"); } } catch (Exception $e) { echo $e->getMessage(); } -> Option A

4. Quick Check:

   Throw inside try, catch prints message [OK]

Hint: Always throw exceptions inside try blocks to catch them [OK]

Common Mistakes:

• Throwing outside try block
• Missing 'new' keyword when throwing
• Printing hardcoded message instead of exception message