10 Most Common PHP Issues and How to Fix Them

PHP is one of the most popular server-side scripting languages, but developers often encounter recurring issues. In this post, we'll explore the top 10 most common PHP issues and their solutions to help you troubleshoot your code effectively.

1. "Undefined Index/Variable" Error

Cause: Accessing an undefined variable or array index.

Solution:

// Use isset() or array_key_exists() to check existence
if (isset($_POST['username'])) {
    $username = $_POST['username'];
} else {
    $username = 'Guest';
}

2. "Headers Already Sent" Error

Cause: Output (e.g., echo, whitespace) sent before using header() or setcookie().

Solution:

• Ensure no output before the header function call.
• Example:

// Place this at the very top of the file
header("Location: another-page.php");

3. "Call to Undefined Function" Error

Cause: Function not included, typo in the function name, or missing PHP extension.

Solution:

• Check the function spelling.
• Ensure the required PHP extension is enabled in php.ini.

// Example: Enable MySQLi
extension=mysqli

4. Session Issues (Not Persisting)

Cause: session_start() not called or improper cookie settings.

Solution:

session_start();
$_SESSION['user'] = 'John';

5. File Upload Not Working

Cause: php.ini settings, file size limits, or incorrect form configuration.

Solution:

• Update php.ini:

upload_max_filesize = 10M
post_max_size = 12M

• Ensure your form includes enctype="multipart/form-data".

6. SQL Injection Vulnerability

Cause: Using unprepared SQL queries with user input.

Solution:

// Use prepared statements
$stmt = $pdo->prepare("SELECT * FROM users WHERE email = :email");
$stmt->execute(['email' => $_POST['email']]);

7. cURL Not Working

Cause: cURL extension not enabled.

Solution:

• Enable cURL in php.ini:

extension=curl

• Example cURL request:

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://api.example.com");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$response = curl_exec($ch);
curl_close($ch);

8. Composer Issues (Autoload Not Working)

Cause: Missing vendor/autoload.php.

Solution:

• Install dependencies with:

composer install

• Ensure correct inclusion:

require 'vendor/autoload.php';

9. Permission Errors (File or Directory)

Cause: Incorrect file or directory permissions.

Solution:

chmod -R 755 /path/to/directory
chown -R www-data:www-data /path/to/directory

10. Fatal Error: Maximum Execution Time Exceeded

Cause: Script takes longer than allowed.

Solution:

// Increase execution time
ini_set('max_execution_time', 300); // 5 minutes

Alternatively, update the max_execution_time setting in php.ini.

Final Thoughts

Encountering errors while working with PHP is inevitable, but with a systematic approach, these can be resolved efficiently. Bookmark this guide for quick reference, and feel free to share your feedback or other common issues in the comments section below!