PHP Error Handling and Debugging

# Chapter 26: PHP Error Handling and Debugging

1. Introduction #

2. Learning Objectives #

• Understand the different levels of PHP Errors (Notices, Warnings, Fatal Errors).

• Enable and disable error reporting.

• Use try/catch blocks to handle Exceptions gracefully without crashing the script.

• Use vardump() and printr() effectively for debugging variables.

3. Types of PHP Errors #

• Notices: Minor issues. The script continues running. (e.g., Echoing a variable that hasn't been defined).

• Warnings: Serious issues, but the script still continues. (e.g., Using include for a file that does not exist).

• Fatal Errors: Critical issues. The script halts execution immediately. (e.g., Calling a function that doesn't exist, or a syntax error like a missing semicolon).

4. Error Reporting #

You can control this at the top of your script (or preferably in your php.ini file):

<?php
// Display all errors (Use ONLY in development!)
ini_set(&#039;display_errors', 1);
ini_set(&#039;display_startup_errors', 1);
error_reporting(E_ALL);

// Example of a Warning
include &#039;missing_file.php'; 
echo "Script still running!";
?>

5. Exceptions and Try/Catch #

<?php
function divideNumbers($a, $b) {
    if ($b == 0) {
        // We manually "throw" an Exception
        throw new Exception("Division by zero is mathematically impossible!");
    }
    return $a / $b;
}

// Handling the Exception
try {
    // Try to run this code
    echo divideNumbers(10, 0);
    echo "This line will not print if an exception is thrown.";
    
} catch (Exception $e) {
    // If it fails, catch the error and do something graceful
    echo "An error occurred: " . $e->getMessage();
}

echo "<br>The script continues running normally down here!";
?>

6. Debugging Basics #

• echo: Good for simple strings/numbers.

• printr($array): Great for printing arrays in a readable format.

• vardump($variable): The ultimate debugging tool. It outputs the variable type, length, and exact value.

• die() / exit(): Stops the script immediately. Used to check the state of data at a specific line.

<?php
$user = ["name" => "John", "age" => 25];

echo "<pre>"; // Makes array output cleanly formatted
var_dump($user);
echo "</pre>";

die("Script stopped here for debugging.");
?>

7. Real-World Examples #

8. Output Explanations #

9. Common Mistakes #

• Syntax Errors: Missing a semicolon ; or a closing bracket }. PHP will often say the error is on line 20, but the missing semicolon is actually at the end of line 19!

• Displaying errors in production: Never leave iniset('displayerrors', 1); on a live site. Hackers will deliberately cause errors to read your file paths.

• Empty Catch Blocks: Writing catch (Exception $e) { } and leaving it empty. The error is silenced, but the script is broken, making it impossible to debug why it's not working. Always log the error!

10. Best Practices #

• Develop with errorreporting(EALL); turned on so you catch Notices and Warnings before they become bigger problems.

• Log production errors to a secure text file using errorlog("Error message");.

1. 2. Fix the error. Then create an array and use vardump() inside <pre> tags to inspect it.

1. 3. Write a try/catch block that connects to a non-existent MySQL database and gracefully echoes "Database Offline."

12. Mini Project: Debugging Exercise App #

Broken Code:

<?php
$grades = [80, 90, "A", 100];
$total = 0;

foreach ($grade in $grades) {
    $total = $total + $grade;
}

$average = $total / count($grade);
echo "Average is: " . $average;
?>

Fixed Code (with explanations):

<?php
$grades = [80, 90, "A", 100];
$total = 0;
$valid_count = 0; // Need a counter for valid numbers

// Fix 1: Incorrect foreach syntax. It is $grades as $grade
foreach ($grades as $grade) {
    // Fix 2: Check if it's a number to avoid math errors on "A"
    if (is_numeric($grade)) { 
        $total += $grade;
        $valid_count++;
    }
}

// Fix 3: Prevent division by zero if array was empty
try {
    if ($valid_count == 0) {
        throw new Exception("No valid grades to calculate.");
    }
    $average = $total / $valid_count; // Fix 4: Divide by valid count, not count($grade)
    echo "Average is: " . $average;
} catch (Exception $e) {
    echo $e->getMessage();
}
?>

13. Coding Challenges #

14. MCQs with Answers #

2. What is the best debugging function to inspect the exact data type and value of an array? A) echo B) print C) vardump() D) debug() *Answer: C*

18. Next Chapter Recommendation #