Error handling with PDO exceptions in PHP - Step-by-Step Execution

Concept Flow - Error handling with PDO exceptions

Start

↓

Try to connect to DB

↓

PDO throws exception?

No→Use DB connection

Yes↓

Catch exception

↓

Handle error (e.g., show message)

↓

End

The program tries to connect to the database. If an error happens, it catches the exception and handles it gracefully.

Execution Sample

PHP

<?php
try {
  $pdo = new PDO($dsn, $user, $pass);
  echo "Connected!";
} catch (PDOException $e) {
  echo "Error: " . $e->getMessage();
}
?>

This code tries to connect to a database and prints a message if it succeeds or an error message if it fails.

Execution Table

Step	Action	Evaluation	Result
1	Try to create PDO object	PDO constructor called with DSN, user, pass	Throws exception if connection fails, else creates object
2	PDO connection success?	No exception thrown	Proceed to echo 'Connected!'
3	Output	Print 'Connected!'	User sees: Connected!
4	Try to create PDO object	PDO constructor called with wrong DSN	Throws PDOException
5	Catch PDOException	Catch block runs	Print error message with exception text
6	Output	Print error message	User sees: Error: [exception message]

💡 Execution stops after printing success or error message.

Variable Tracker

Variable	Start	After Step 1	After Step 4	Final
$pdo	null	PDO object if success	null (exception thrown)	PDO object or null
$e	undefined	undefined	PDOException object	PDOException object or undefined

Key Moments - 3 Insights

Why do we use try-catch with PDO?

Because PDO throws exceptions on errors, try-catch lets us handle these errors without stopping the whole program. See execution_table rows 1 and 4.

What happens if the connection is successful?

The code inside try runs fully and prints 'Connected!'. No catch block runs. See execution_table rows 2 and 3.

What if the connection fails?

PDO throws an exception, the catch block runs and prints the error message. See execution_table rows 4, 5, and 6.

Visual Quiz - 3 Questions

Look at the execution_table, what variable holds the PDOException object?

A $pdo

B $e

C $dsn

D $user

💡 Hint

Check variable_tracker row for $e after Step 4

At which step does the program print 'Connected!'?

A Step 3

B Step 1

C Step 5

D Step 6

💡 Hint

Look at execution_table row with Output 'Connected!'

If the DSN is wrong, what happens in the try block?

A Nothing happens

B PDO object is created successfully

C Exception is thrown and caught

D Program crashes without message

💡 Hint

See execution_table rows 4 and 5 about exception throwing and catching

Concept Snapshot

try {
  $pdo = new PDO($dsn, $user, $pass); // try to connect
  echo "Connected!"; // success message
} catch (PDOException $e) {
  echo "Error: " . $e->getMessage(); // handle error
}
Use try-catch to catch PDO exceptions and avoid program crash.

Full Transcript

This example shows how to handle errors when connecting to a database using PDO in PHP. The code tries to create a PDO object inside a try block. If the connection is successful, it prints 'Connected!'. If there is a problem, PDO throws an exception. The catch block catches this exception and prints an error message. This way, the program does not crash and shows a friendly message instead.