Set_error_handler function in PHP - Step-by-Step Execution

Concept Flow - Set_error_handler function

Start Program

↓

Call set_error_handler

↓

Error occurs?

No→Continue normal execution

Yes↓

Custom error handler called

↓

Handle error (log, display, etc.)

↓

Return control to program

↓

Program ends

The program sets a custom error handler, then when an error happens, it calls that handler to manage the error before continuing.

Execution Sample

PHP

<?php
function myErrorHandler($errno, $errstr) {
  echo "Error: [$errno] $errstr\n";
}
set_error_handler('myErrorHandler');
echo $undefinedVar;
?>

This code sets a custom error handler that prints error details, then triggers an error by using an undefined variable.

Execution Table

Step	Action	Evaluation	Result
1	Define function myErrorHandler	Function created	Ready to handle errors
2	Call set_error_handler('myErrorHandler')	Set custom error handler	Errors now use myErrorHandler
3	Execute echo $undefinedVar;	Undefined variable error triggered	Call myErrorHandler with error info
4	Inside myErrorHandler	Print error message	Output: Error: [8] Undefined variable: undefinedVar
5	Return from myErrorHandler	Error handled	Program continues
6	Program ends	No more code	Execution complete

💡 Program ends after handling the undefined variable error with custom handler

Variable Tracker

Variable	Start	After Step 3	After Step 5	Final
$errno	N/A	8 (Notice)	8 (Notice)	8 (Notice)
$errstr	N/A	"Undefined variable: undefinedVar"	"Undefined variable: undefinedVar"	"Undefined variable: undefinedVar"
$undefinedVar	Undefined	Undefined error triggered	Undefined error handled	Undefined

Key Moments - 2 Insights

Why does the custom error handler get called when echoing an undefined variable?

Because echoing an undefined variable triggers a notice error, and set_error_handler directs PHP to call the custom handler instead of the default error behavior, as shown in step 3 and 4 of the execution_table.

Does the program stop after the error is handled?

No, after the custom error handler runs (step 4 and 5), the program continues normally until it ends, as shown in step 6.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output at step 4?

A Error: [8] Undefined variable: undefinedVar

B Fatal error: Undefined variable

C No output

D Warning: Variable undefinedVar not set

💡 Hint

Check the 'Result' column at step 4 in the execution_table.

At which step does the custom error handler get set?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Action' column to find when set_error_handler is called.

If the error handler function was not set, what would happen at step 3?

A The program would ignore the error and continue silently

B PHP would use the default error handler and show a notice

C The program would crash immediately

D The error would be logged but not displayed

💡 Hint

Recall that without set_error_handler, PHP uses its default error handling for notices.

Concept Snapshot

set_error_handler(functionName)
- Registers a custom function to handle PHP errors
- When an error occurs, PHP calls this function instead of default
- Function receives error number and message
- Allows custom error display or logging
- Program continues after handler unless stopped

Full Transcript

This example shows how PHP's set_error_handler function works. First, a custom function named myErrorHandler is defined to print error details. Then, set_error_handler is called with this function's name, so PHP uses it for errors. When the code tries to echo an undefined variable, PHP triggers a notice error. Instead of default handling, PHP calls myErrorHandler with error info. The handler prints the error message. After handling, the program continues and ends normally. Variables like $errno and $errstr hold error details during handling. This lets programmers control error messages and behavior in their PHP scripts.