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PHP Errors and Exceptions

Detailed information about basic PHP errors on your Pantheon Drupal or WordPress site.

This section provides information on how to interpret and resolve PHP errors.

There are three basic types of PHP errors:

  • Notice: room for improvement. This is typically caused by unset variables or missing array keys.
  • Warning: errors will probably occur if not addressed.
  • Error: fatal, execution terminated. Often known as the "white screen of death."

Refer to Error Handling and Logging for more information.

Each of the PHP errors are handled differently depending on the site environment. On Dev and Test, errors are shown directly to the user in the browser. On Live, PHP errors are not displayed to users, but are still logged. Notices and warnings are logged in the database logs if db_log is enabled for Drupal. The PHP constants WP_DEBUG and WP_DEBUG_LOG can be enabled for WordPress to save errors to wp-content/debug.log. PHP errors are also logged on the application container at logs/php-error.log.

Here's a breakdown of what errors are shown and where:


















Refer to Log Files on Pantheon for more information.

Performance Hits

An error is a problem that needs to be addressed regardless of severity. Any PHP error will drastically reduce the speed of PHP execution. Even if you don't see the error in your browser, and have explicitly disabled logging, every PHP error will slow your site down.

If database logging is enabled, your site will be even slower, requiring a database write for every error. However, disabling logging does not address the problem, it only hides the symptom.

Best practice is to fix every notice, warning, and error as you discover them. If they're in an extension (WordPress plugin or Drupal module), roll a patch and submit it to the project's issue queue.

Refer to this stackoverflow thread for more details, including benchmarks that compare the differences between suppressing notices and actually eliminating the root cause.

Unhandled Exceptions

A PHP exception is a mechanism for defining error conditions and how to handle them. Refer to PHP documentation on Exceptions for more information.

PHP Exceptions are errors, and depending on the severity can crash your site. As Exceptions are created in code and not by PHP itself, they are not logged in the PHP error log file. By default, Drupal will log exceptions to Watchdog.

Undefined Function Error

Normally a request to Drupal or WordPress starts by reading the index.php file at the root directory, which then bootstraps

the site.

However, when a PHP file is requested directly (for example,, the index.php file and the bootstrap process are skipped. Instead, the PHP file is executed on it's own and any function included but not defined by the file will cause a Call to undefined function fatal error.

For example, the following error PHP reports the phpfile.php file at line xx for calling some_function(), which has not yet been defined: Call to undefined function [some_function()] in [path/to/phpfile.php:xx]

Troubleshooting Undefined Function Errors

Use the following debugging techniques to investigate undefined function error messages:

  • Search for the offending function elsewhere in the codebase to make sure it's defined somewhere in the project.
  • Check the reported PHP file at full bootstrap. Rather than accessing the reported PHP file directly (for example,, browse to a page that includes the file (for example, to see if the same error occurs with the site fully bootstrapped.
  • Review nginx-access.log for requests to the reported PHP file.

If you see direct requests to PHP files causing fatal undefined function errors (often caused by bot traffic), use the pantheon.yml configuration file to set protected web paths. Refer to Pantheon YAML Configuration Files for more information. This stops the file from being web accessible, while keeping the file available to PHP during bootstrap. When accessed directly, protected paths and files return a 403 Access Denied server response.

We also recommend submitting a sitemap and instructing bots to only crawl designated paths set in robots.txt. Refer to Bots and Indexing on Pantheon for more information.

Undefined Index Notices

PHP notices may be reported on your Dev site that have never been reported before when you import your site or enable some new modules. These notices are now apparent because of the Dev environment's strict error reporting level.

An example notice might look like this:

Notice: Undefined index: description in theme_imagefield_image_imagecache_lightbox2() (line 163 of /srv/bindings/xxxxxxxxx/code/sites/all/modules/contrib/lightbox2/

Why is PHP reporting this?

Variable declaration is not required by PHP, but is a recommended practice that can help to avoid security vulnerabilities or bugs if one forgets to provide a value to a variable that be used later on. PHP issues an E\_NOTICE, a very low-level error, as a reminder.

No one is going to twist your arm about addressing these notices, but Pantheon believes that surfacing them in the Development environment will help developers address potential problems in the future before they can occur by following best practices.

Fatal Error: require_once(): Failed Opening Required

The require\_once() function simply checks to see if a file has been included already. If it has not, then it will be included when checked.

When this error surfaces, it simply means that the file in question is not where it should be. For example, the error will look something like this:

Fatal error: require_once(): Failed opening required ‘/srv/bindings/xxxxx/code/sites/all/modules/redis/’ (include_path=.:/usr/share/pear:/usr/share/php’) in /srv/bindings/xxxxxx/code/includes/ on line 2394

Look for the correct path to the file and update the require\_once() to fix this error.

Intermittent Notices

Refer to Debug Intermittent PHP 7 Notices for additional information if you are encountering intermittent notices that are not behaving as described on this page. This is not common behavior and you should try to resolve this immediately.

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