Set_error_handler function in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP code using set_error_handler?

Consider the following PHP code that sets a custom error handler. What will it output when run?

PHP

<?php
function customError($errno, $errstr) {
    echo "Error caught: [$errno] $errstr\n";
}
set_error_handler('customError');
// Trigger an error
echo $undefinedVar;
?>

A Error caught: [8] Undefined variable: undefinedVar

B Notice: Undefined variable: undefinedVar in script on line 7

C No output

D Fatal error: Uncaught Error: Undefined variable: undefinedVar

💡 Hint

The custom error handler replaces the default error message for notices.

The set_error_handler function sets a user-defined function to handle errors. When the undefined variable is accessed, a notice is triggered. Instead of the default PHP notice, the customError function runs and outputs the formatted message.

🧠 Conceptual

intermediate

Which error types can be handled by set_error_handler?

In PHP, which of the following error types can be caught and handled by a function set with set_error_handler?

A All errors including fatal compile-time errors

B E_NOTICE, E_WARNING, E_USER_ERROR

C E_COMPILE_ERROR, E_CORE_WARNING, E_PARSE

D E_PARSE, E_ERROR, E_CORE_ERROR

💡 Hint

Some errors occur before the script runs and cannot be caught.

set_error_handler can handle runtime errors like notices, warnings, and user-generated errors. It cannot handle parse errors or fatal errors that stop script execution before the handler is set.

🔧 Debug

advanced

Why does this custom error handler not catch the error?

Look at this PHP code. The custom error handler is set, but the error is not caught by it. Why?

PHP

<?php
function myErrorHandler($errno, $errstr) {
    echo "Caught error: $errstr\n";
}
// Intentionally trigger a parse error
set_error_handler('myErrorHandler');
eval('echo "Hello"');
?>

A Parse errors cannot be caught by set_error_handler because they occur before runtime.

B The error handler function name is incorrect.

C eval() suppresses all errors by default.

D The error handler must be set after the eval statement.

💡 Hint

Parse errors happen before the script runs.

Parse errors happen during the compilation phase before the script runs, so set_error_handler cannot catch them. Only runtime errors can be caught.

📝 Syntax

advanced

Which option correctly sets a custom error handler that ignores E_NOTICE errors?

You want to set a custom error handler in PHP that handles all errors except notices (E_NOTICE). Which code snippet does this correctly?

A set_error_handler('handler', E_NOTICE);

B set_error_handler('handler', E_ALL | E_NOTICE);

C set_error_handler('handler', E_ALL & ~E_NOTICE);

D set_error_handler('handler', E_ALL ^ E_NOTICE);

💡 Hint

Use bitwise operators to exclude E_NOTICE.

E_ALL & ~E_NOTICE means all errors except notices. The bitwise AND with the negation of E_NOTICE excludes notices from the handled errors.

🚀 Application

expert

What is the output of this PHP code with nested error handlers?

Analyze this PHP code that sets two error handlers one after another. What will be the output when an undefined variable is accessed?

PHP

<?php
function firstHandler($errno, $errstr) {
    echo "First handler: $errstr\n";
    return false; // Pass to next handler
}
function secondHandler($errno, $errstr) {
    echo "Second handler: $errstr\n";
}
set_error_handler('firstHandler');
set_error_handler('secondHandler');
echo $undefinedVar;
?>

A No output

B First handler: Undefined variable: undefinedVar

C

First handler: Undefined variable: undefinedVar
Second handler: Undefined variable: undefinedVar

D Second handler: Undefined variable: undefinedVar

💡 Hint

The last set_error_handler replaces the previous one.

Only the last error handler set is active. The firstHandler is replaced by secondHandler. So only secondHandler runs and outputs the 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