Throwing exceptions in PHP - Step-by-Step Execution

Concept Flow - Throwing exceptions

Start

↓

Code runs

↓

Condition triggers error?

No→Continue normal flow

Yes↓

Throw exception

↓

Catch exception?

No→Program stops with error

Yes↓

Handle exception

↓

Continue or exit gracefully

The program runs code, checks for error conditions, throws an exception if needed, then either catches and handles it or stops.

Execution Sample

PHP

<?php
try {
  if ($x < 0) {
    throw new Exception("Negative value");
  }
  echo "Value is $x";
} catch (Exception $e) {
  echo $e->getMessage();
}

This code throws an exception if $x is negative and catches it to print the error message.

Execution Table

Step	Code Line	Condition	Action	Output
1	$x = -1;	N/A	Assign -1 to $x
2	if ($x < 0)	$x = -1 < 0 is True	Throw Exception
3	throw new Exception	Exception thrown	Jump to catch block
4	catch (Exception $e)	Catch exception	Handle exception
5	echo $e->getMessage()	Print message	Output: Negative value	Negative value

💡 Exception caught and handled, program continues after catch block

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
$x	undefined	-1	-1	-1	-1	-1
$e	undefined	undefined	undefined	Exception object	Exception object	Exception object

Key Moments - 3 Insights

Why does the program jump directly to the catch block after throwing the exception?

When the exception is thrown at step 3, normal code stops and control jumps to the catch block at step 4 to handle the error.

What happens if there is no catch block for the thrown exception?

If no catch block exists, the program stops with a fatal error because the exception is unhandled.

Why is the output 'Negative value' instead of 'Value is -1'?

Because the exception is thrown before the echo statement, the normal output is skipped and the catch block prints the error message.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of $x after step 2?

A -1

B 0

C undefined

D Exception object

💡 Hint

Check the variable_tracker row for $x after step 2

At which step does the program jump to the catch block?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the action column in the execution_table for when the exception is thrown

If $x was 5 instead of -1, what would happen at step 2?

A Exception would be thrown

B Condition would be false, no exception thrown

C Catch block would run

D Program would stop with error

💡 Hint

Check the condition column at step 2 and think about $x = 5

Concept Snapshot

Throw exceptions with throw new Exception("message");
Use try { } catch (Exception $e) { } to handle.
If exception thrown, normal flow stops and jumps to catch.
Without catch, program stops with error.
Catch block can access error message with $e->getMessage().

Full Transcript

This visual trace shows how throwing exceptions works in PHP. The program assigns a value to $x. It checks if $x is less than zero. If yes, it throws an exception. Throwing stops normal code and jumps to the catch block. The catch block handles the error by printing the message. Variables $x and $e change as the program runs. If no exception is thrown, the program prints the value normally. This helps handle errors cleanly without crashing.

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