Set_error_handler function in PHP - Mini Project: Build & Apply

Using set_error_handler in PHP

📖 Scenario: Imagine you are building a PHP script that needs to handle errors in a custom way. Instead of PHP showing default error messages, you want to catch errors and display a friendly message or log them differently.

🎯 Goal: You will create a custom error handler function and use set_error_handler to tell PHP to use your function when an error happens.

📋 What You'll Learn

Create a custom error handler function named myErrorHandler that accepts the standard error parameters.

Use set_error_handler to set myErrorHandler as the active error handler.

Trigger a warning error using trigger_error to test the custom handler.

Print the custom error message from inside the error handler.

💡 Why This Matters

🌍 Real World

Custom error handlers help websites and applications show friendly messages or log errors in a controlled way instead of default PHP errors.

💼 Career

Knowing how to manage errors is important for PHP developers to build robust, user-friendly applications and maintain code quality.

1

Create the custom error handler function

Write a function called myErrorHandler that accepts four parameters: $errno, $errstr, $errfile, and $errline. Inside the function, use echo to print the string: "Custom error: " followed by the error message $errstr and a newline character.

PHP

# Write the function myErrorHandler with parameters $errno, $errstr, $errfile, $errline
# Inside, print "Custom error: " followed by $errstr and a newline

Hint

Define a function named myErrorHandler with the four error parameters. Use echo to print the message.

2

Set the custom error handler

Use set_error_handler to set myErrorHandler as the active error handler.

PHP

<?php
function myErrorHandler($errno, $errstr, $errfile, $errline) {
    echo "Custom error: $errstr\n";
}

# Use set_error_handler to set myErrorHandler as the error handler

Hint

Call set_error_handler with the string 'myErrorHandler' as argument.

3

Trigger a warning error

Use trigger_error to trigger a user warning with the message 'This is a test warning.'.

PHP

<?php
function myErrorHandler($errno, $errstr, $errfile, $errline) {
    echo "Custom error: $errstr\n";
}

set_error_handler('myErrorHandler');

# Trigger a warning error with message 'This is a test warning.'

Hint

Call trigger_error with the message and E_USER_WARNING as parameters.

4

Print the custom error message

Run the script and observe that the output shows the custom error message starting with Custom error: This is a test warning.

PHP

<?php
function myErrorHandler($errno, $errstr, $errfile, $errline) {
    echo "Custom error: $errstr\n";
}

set_error_handler('myErrorHandler');

trigger_error('This is a test warning.', E_USER_WARNING);

# Run the script to see the output

Hint

Run the PHP script. The output should show the custom error message.

Practice

1. What is the main purpose of the set_error_handler function in PHP?

easy

A. To define a custom function to handle errors when they occur

B. To stop all errors from appearing

C. To automatically fix errors in the code

D. To log errors to a file without any custom handling

Solution

1. Step 1: Understand what set_error_handler does

   This function lets you tell PHP to use your own function to handle errors instead of the default error messages.

2. Step 2: Compare options with this understanding

   Only defining a custom function to handle errors when they occur correctly describes this behavior. Automatically fixing errors, stopping all errors from appearing, and logging errors to a file without any custom handling are all incorrect.

3. Final Answer:

   To define a custom function to handle errors when they occur -> Option A

4. Quick Check:

   Custom error handling = Define function with set_error_handler [OK]

Hint: Remember: set_error_handler sets your own error function [OK]

Common Mistakes:

• Thinking it stops errors completely
• Assuming it fixes errors automatically
• Confusing it with error logging functions

2. Which of the following is the correct syntax to set a custom error handler function named myErrorHandler?

easy

A. set_error_handler(&myErrorHandler);

B. set_error_handler(myErrorHandler);

C. set_error_handler('myErrorHandler');

D. set_error_handler($myErrorHandler);

Solution

1. Step 1: Recall the syntax for passing function names in PHP

   When passing a function name as a callback, it must be a string in quotes.

2. Step 2: Check each option

   set_error_handler('myErrorHandler'); correctly passes the function name as a string. set_error_handler(myErrorHandler); misses quotes, set_error_handler(&myErrorHandler); uses an invalid reference operator, and set_error_handler($myErrorHandler); uses a variable which is incorrect here.

3. Final Answer:

   set_error_handler('myErrorHandler'); -> Option C

4. Quick Check:

   Function name as string = 'myErrorHandler' [OK]

Hint: Function names as strings in callbacks [OK]

Common Mistakes:

• Omitting quotes around function name
• Using variable instead of string
• Using reference operator & incorrectly

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

function customError($errno, $errstr) {
    echo "Error: [$errno] $errstr";
}
set_error_handler('customError');
trigger_error('This is a test error', E_USER_WARNING);

medium

A. Warning: This is a test error

B. Error: [512] This is a test error

C. Fatal error: Uncaught Error

D. No output

Solution

1. Step 1: Understand the custom error handler and trigger_error

   The customError function prints the error number and message. The trigger_error calls this handler with E_USER_WARNING which has error number 512.

2. Step 2: Predict the output

   The output will be the echo from customError showing the error number and message exactly as coded.

3. Final Answer:

   Error: [512] This is a test error -> Option B

4. Quick Check:

   Custom handler prints error number 512 and message [OK]

Hint: E_USER_WARNING error number is 512 [OK]

Common Mistakes:

• Expecting default PHP warning format
• Confusing error numbers
• Thinking no output appears

4. Identify the error in this PHP code snippet using set_error_handler:

function handler($errno, $errstr) {
    echo "Error: $errstr";
}
set_error_handler(handler);
trigger_error('Oops!');

medium

A. trigger_error requires two arguments

B. trigger_error cannot be used with set_error_handler

C. handler function must return a value

D. Missing quotes around handler in set_error_handler

Solution

1. Step 1: Check the set_error_handler syntax

   The function name passed to set_error_handler must be a string, so it needs quotes.

2. Step 2: Verify trigger_error usage

   trigger_error can be called with one argument; the second is optional. The handler function does not need to return a value. Using trigger_error with set_error_handler is valid.

3. Final Answer:

   Missing quotes around handler in set_error_handler -> Option D

4. Quick Check:

   Function name must be string in quotes [OK]

Hint: Function names need quotes in set_error_handler [OK]

Common Mistakes:

• Omitting quotes around function name
• Thinking trigger_error needs two arguments
• Expecting handler to return a value

5. You want to log all warnings and notices to a file instead of displaying them. Which approach using set_error_handler is best?

hard

A. Create a function that writes error details to a log file and set it with set_error_handler

B. Use set_error_handler with an empty function to ignore errors

C. Call trigger_error inside set_error_handler to log errors

D. Disable error reporting and rely on PHP default logging

Solution

1. Step 1: Understand the goal

   You want to capture warnings and notices and save them to a file instead of showing them on screen.

2. Step 2: Evaluate options

   Create a function that writes error details to a log file and set it with set_error_handler correctly describes creating a custom function that writes error info to a log file and setting it as the error handler. Use set_error_handler with an empty function to ignore errors ignores errors, which is not logging. Call trigger_error inside set_error_handler to log errors misuses trigger_error inside the handler, causing recursion. Disable error reporting and rely on PHP default logging disables reporting but does not customize logging.

3. Final Answer:

   Create a function that writes error details to a log file and set it with set_error_handler -> Option A

4. Quick Check:

   Custom handler writes logs = best practice [OK]

Hint: Custom handler writes logs to file for error control [OK]

Common Mistakes:

• Ignoring errors instead of logging
• Calling trigger_error inside handler causing loops
• Disabling errors without custom logging