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Troubleshooting Common PHP Warnings in WP Super Cache

44 threads Sep 10, 2025 Plugin → Wp super cache

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If you're seeing PHP warnings or notices in your debug logs related to WP Super Cache, you're not alone. These messages are a common side effect of the plugin's evolution to maintain compatibility with newer versions of PHP, especially PHP 7.4 and above. While often harmless, they can be annoying and sometimes point to a configuration issue.

This guide will walk you through the most frequently reported warnings and how to resolve them.

Common Warnings and Their Fixes

1. Warning: Invalid argument supplied for foreach() in wp-cache.php on line 2050

This is one of the most common warnings. It occurs when the "Rejected Cookies" field in the plugin's advanced settings is left completely empty.

Solution: Navigate to WP Super Cache -> Advanced -> Rejected Cookies. If the field is blank, enter a single dummy value like "test" or simply click the "Save" button on that form. This action initializes the setting and prevents the warning.

2. Warning: Trying to access array offset on value of type null / Undefined offset

These warnings, often found in `wp-cache-phase2.php`, typically indicate that a variable expected to be an array is instead `null` or undefined when the code tries to use it. This is a strict type check introduced in PHP 7.x and 8.x.

Solution: These are generally considered harmless and related to internal plugin functions. The WP Super Cache team has been actively fixing these in recent updates. Ensure you are running the latest version of the plugin. If the warning persists, it may be a conflict with another plugin, such as an e-commerce or review plugin that hooks into comments.

3. Warning: Constant WPSC_VERSION already defined

This error happens when another plugin or theme defines a constant with the same name (e.g., e-commerce plugins or the Support Candy plugin).

Solution: This is a known issue, and the WP Super Cache team has stated they will change this constant in a future release. As a temporary workaround, you can deactivate one of the conflicting plugins to identify the culprit.

4. Warning: Cannot modify header information - headers already sent

This critical error is almost never caused by WP Super Cache itself. It means output (like whitespace, text, or a UTF-8 BOM) is being sent to the browser before WordPress can send HTTP headers. The plugin file mentioned is just where the process fails.

Solution: The cause is usually a theme or plugin file. Check the file mentioned in the first part of the error message (e.g., `wp-config.php`, `iffeed-scotland.php`, `nav-desktop.php`). Ensure the file is saved without a leading Byte Order Mark (BOM) and that there is no whitespace or text outside of the opening <?php and closing ?> tags.

5. Deprecated Function Warnings (e.g., str_replace(), is_dir(), substr())

PHP 8.1+ started emitting "deprecated" notices when functions receive `null` where they expect a string. Many of these, such as the `str_replace()` warning on line 54, have been fixed in recent versions of WP Super Cache (1.10.0 and above).

Solution: Update to the latest version of WP Super Cache. If you are up-to-date and still see a deprecated warning, report it to the plugin's support forums, as it is likely a newly identified issue slated for a future patch.

General Troubleshooting Steps

  • Update Everything: First, always ensure WordPress, your theme, all plugins, and especially WP Super Cache are updated to their latest versions.
  • Conflict Test: If a warning persists, deactivate all other plugins and switch to a default theme like Twenty Twenty-Four. If the warning disappears, reactivate your plugins one by one to find the conflict.
  • Check Your Logs: For non-critical warnings, you can often disable displaying errors to users by setting WP_DEBUG_DISPLAY to false in your `wp-config.php` file. The errors will still be logged for your review.

Most of these warnings do not affect the core functionality of the WP Super Cache plugin. The development team is consistently working to improve PHP compatibility, so keeping the plugin updated is the best long-term strategy for a warning-free experience.

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