The Nginx 500 Internal Server Error occurs when Nginx successfully receives a request, but the application encounters an unexpected error.

This is commonly caused by application errors, PHP memory exhaustion, missing application files, or incorrect file permissions.

In this guide, you’ll learn how to identify the root cause using Nginx logs and fix the most common causes of the error.

How to Troubleshoot and Fix Nginx 500 Internal Server Error

Start by checking the Nginx error log to identify the root cause, then follow the appropriate troubleshooting steps below.

1. Backend Application Errors
2. PHP Syntax Errors
3. PHP Memory Exhaustion
4. File Permission Issues
5. Missing Required Files or Dependencies

1. Fix Backend Application Errors

Backend application failures are one of the most common causes of an Nginx 500 Internal Server Error.

These errors occur when the application encounters an unexpected error, such as calling an undefined function, loading a missing dependency, or failing while executing the application code.

How to Identify

Check the Nginx error log:

sudo tail -f /var/log/nginx/error.log

Example Error:

PHP Fatal error: Call to undefined function

Solution

Review the application code and identify the error reported in the Nginx log.

• Fix application code errors.
• Verify all required libraries and dependencies are installed.
• Review recent deployments or configuration changes.

If required, restart PHP-FPM after applying the fix:

sudo systemctl restart php8.1-fpm

OR

sudo systemctl restart php8.2-fpm

Note: A PHP fatal error stops the application from processing the request. When this happens, Nginx returns a 500 Internal Server Error

2. Fix PHP Syntax Errors

A PHP syntax error occurs when the PHP interpreter cannot parse the application code due to invalid syntax, such as missing semicolons, unmatched braces, missing quotes, or incorrect function definitions.

How to Identify

Check the Nginx error log:

sudo tail -f /var/log/nginx/error.log

Example Error:

PHP Parse error: syntax error

Solution

• Check for missing semicolons, brackets, or quotes.
• Verify that all functions and statements are correctly defined.
• Validate the PHP syntax before deploying the application.

Use the following command to check the syntax of a PHP file:

php -l index.php

Note: The php -l command checks the PHP syntax without executing the script, making it a quick way to identify syntax errors before deployment.

3. Fix PHP Memory Exhaustion

Applications may return an Nginx 500 Internal Server Error when they exceed the configured PHP memory limit. This commonly occurs during large file uploads, report generation, image processing, or inefficient application code.

How to Identify

Check the Nginx error log:

sudo tail -f /var/log/nginx/error.log

Example Error:

PHP Fatal error:  Allowed memory size of 134217728 bytes exhausted

Solution

Step 1: Check the PHP CLI memory limit.

Run the following command to check the memory limit configured for the PHP command-line interface (CLI):

php -i | grep memory_limit

Example output:

memory_limit => -1 => -1

Note: The php -i command displays the PHP CLI configuration. Nginx processes web requests using PHP-FPM, which may use a different php.ini file and memory limit.

Step 2: Update the PHP-FPM memory limit.

Open the PHP-FPM configuration file:

sudo nano /etc/php/<version>/fpm/php.ini

Search for:

memory_limit = 128M

Increase the value if required. For example:

memory_limit = 256M

Step 3: Restart PHP-FPM

Save the file and restart PHP-FPM service.

For PHP 8.1:

sudo systemctl restart php8.1-fpm

For PHP 8.2:

sudo systemctl restart php8.2-fpm

Step 4: Verify the new memory limit

Create a temporary PHP file:

<?php
phpinfo();
?>

Open the file in your browser and search for memory_limit. The displayed value reflects the memory limit used by PHP-FPM for web requests.

4. Fix File Permission Issues

Incorrect file or directory permissions can cause an Nginx 500 Internal Server Error when the application cannot read, write, or modify required files. This commonly occurs after deployments, permission changes, or incorrect file ownership.

How to Identify

Check the Nginx error log:

sudo tail -f /var/log/nginx/error.log

Example Error:

PHP Warning: fopen(): Failed to open stream: Permission denied
PHP Fatal error: Uncaught TypeError: fwrite(): Argument #1 ($stream) must be of type resource, bool given

Check the ownership and permissions of the affected file or directory:

ls -ld /path/to/directory 
ls -l /path/to/file

Check the PHP-FPM user:

grep -E '^(user|group) =' /etc/php/8.1/fpm/pool.d/www.conf

On a default Ubuntu PHP-FPM installation, this is commonly:

user = www-data 
group = www-data

Solution

Make sure the PHP-FPM user has the required access to the application files.

Check the current ownership:

ls -ld /path/to/directory

Update the ownership if required:

sudo chown -R www-data:www-data /path/to/directory

Update the permissions if required:

sudo chmod -R 755 /path/to/directory

After correcting the ownership or permissions, access the application again and verify that the 500 error is resolved.

5. Fix Missing Required Files or Dependencies

Missing required application files or dependencies can cause an Nginx 500 Internal Server Error when the application tries to load a file that does not exist. This commonly occurs after incomplete deployments, accidental file deletion, failed application upgrades, or CI/CD packaging issues.

How to Identify

Check the Nginx error log:

sudo tail -f /var/log/nginx/error.log

Example Error:

PHP Warning: require(database.php): Failed to open stream: No such file or directory 
PHP Fatal error: Failed opening required 'database.php'

Solution

Step 1: Verify the missing file

Check whether the required file exists on the server.

ls -l /var/www/html/database.php

Common missing files include:

vendor/autoload.php 
.env 
config.php 
database.php

Step 2: Restore the missing file

If the file is missing, restore it from one of the following sources:

• Source control (Git)
• Deployment artifact
• Application package
• Backup

Step 3: Verify the application

Access the application again and confirm that the 500 Internal Server Error has been resolved.

If the application uses opcode caching or application-level caching and the change is not reflected, restart PHP-FPM:

sudo systemctl restart php8.1-fpm

OR

sudo systemctl restart php8.2-fpm

What is Nginx 500 Internal Server Error?

The Nginx 500 Internal Server Error means that the server encountered an unexpected error while processing the request.

When Nginx is used with PHP applications, the error is commonly caused by the application, such as PHP errors, memory exhaustion, incorrect file permissions, or missing application files.

Unlike a 502 Bad Gateway or 503 Service Temporarily Unavailable, a 500 error generally indicates that the request reached the application, but the application could not complete it.

Best Practices to Prevent Nginx 500 Internal Server Errors

Follow these practices to reduce the chances of Nginx 500 Internal Server Errors:

• Test application changes before deploying them to production.
• Validate PHP syntax using php -l before deployment.
• Include all required application files and dependencies in the deployment package.
• Verify file ownership and permissions after deployments.
• Set an appropriate PHP memory limit based on the application’s requirements.
• Review Nginx and application logs regularly to catch errors early.
• Test the application after every deployment to verify that requests are being processed correctly.

Conclusion

An Nginx 500 Internal Server Error usually means that the application encountered an error while processing a request.

Start by checking the Nginx error log to identify the root cause, then apply the appropriate fix instead of making random configuration changes.

This approach helps you quickly troubleshoot common issues such as PHP errors, memory exhaustion, file permissions, and missing application files.