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.