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Fix the Cannot Modify Header Information Warning in WordPress

WordPress powers many small-business, membership and WooCommerce sites across Australia. When “Cannot modify header information — headers already sent by” appears during login, checkout or a privacy-tool redirect, it may look like a WordPress core fault. It is usually a PHP output-order problem, and dismissing it as a cosmetic browser error can leave redirects, cookies and authentication flows working incorrectly.

Start with the first warning rather than the last message on screen. The named file and line usually identify where PHP sent output before a header could be created, while the destination URL in the network log shows which redirect or plugin was involved. That evidence is especially useful for Australian teams maintaining sites across Sydney, Melbourne, Brisbane and Perth, where shared-hosting nodes, caching layers and daylight-saving changes can make a fault appear at different times.

How the Warning Happens

The warning comes from PHP’s header system when code tries to send a redirect, cookie or other HTTP header after the response body has begun. header(), setcookie() and WordPress functions such as wp_redirect() therefore fail. “Headers already sent” is the important clue: something printed a space, line break, notice or complete response before the intended header call. The file mentioned after “sent by” is the likely source, not automatically the plugin named in a redirect notice.

That distinction prevents destructive troubleshooting. Replacing every exit with return, deleting security headers or enabling permanent output buffering can conceal the fault while leaving broken redirects or exposed sessions behind. Check whether the page still returns the expected 301 or 302 status and Location header. A warning near the top of a page may be harmless visually, but a failed redirect can produce a loop, an expired login or a checkout that silently returns customers to the same step.

Find the First PHP Error

Enable detailed logging on a staging copy or during a controlled maintenance window. In wp-config.php, add these constants if equivalent settings are not already present, and keep them before WordPress is loaded. Edit an existing definition rather than adding a duplicate constant.

define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );
define( 'WP_DEBUG_DISPLAY', false );

WP_DEBUG_LOG records errors in wp-content/debug.log while WP_DEBUG_DISPLAY keeps them out of the rendered page. Search that file for headers already sent and note the earliest timestamp and file path. The first event normally exposes the original output; later entries often repeat the same cause. If shell access is available, grep -n "headers already sent" wp-content/debug.log narrows the results quickly. Otherwise, open the log through the hosting panel or download it with SFTP.

Also inspect the PHP and web-server error logs, not only WordPress. A shared Australian host may place them in separate directories for Apache or LiteSpeed, while Nginx installations commonly use a server-specific log. Ask support for the log covering the exact request time if necessary. Record the time in UTC and convert it to AEST or AEDT; daylight-saving transitions can otherwise make a scheduled plugin task look unrelated to the first redirect attempt.

Repair Files and Plugin Code

The most common file-level cause is a stray blank line or character after PHP’s closing tag. In a PHP-only file, remove the closing ?> entirely; in a mixed template, make sure the closing tag starts a fresh line with no preceding spaces. A UTF-8 byte-order mark at the beginning of wp-config.php, a theme file or plugin PHP file can create the same symptom even though nothing is visible in the editor. Re-save affected files as UTF-8 without BOM, then confirm that the byte shown in the warning matches the corrected file.

For plugin and theme faults, use a recent backup to compare the suspect file with the distributed version. Deactivate extensions one at a time in staging, beginning with security, membership, redirect, cache and WooCommerce shipping tools. A plugin that prints diagnostics, starts output during init or sends a cookie before template_redirect can disrupt a later redirect. Updating a maintained extension may resolve the problem; a custom patch should preserve the original hook priority and avoid adding output before the exit point.

Australian WooCommerce stores often combine an Australia Post or courier extension, a local payment gateway and a page-cache plugin, so removing one component can affect live checkout behaviour. Test the complete customer path rather than assuming every plugin is equally responsible. The WordPress Q&A guides can help cross-check common plugin and redirect patterns before code is changed on a busy shop.

Resolve Redirect and Cache Conflicts

Redirects must run before WordPress starts template output, and each redirect must terminate the current request. For example, a membership rule belongs on an action such as template_redirect, followed by exit; two plugins should not both try to redirect the same request.

add_action( 'template_redirect', function () {
    if ( is_page( 'members-only' ) ) {
        wp_safe_redirect( home_url( '/login/' ) );
        exit;
    }
} );

Use wp_safe_redirect() for a local destination and wp_redirect() only when you intentionally allow an external URL. Give the correct status code, such as 301 for a permanent move or 302 for a temporary one, and keep the exit immediately after the call. Remove or defer any old header() call that emits the same Location value. This prevents duplicate warnings and stops a second callback from replacing the intended destination.

Cache plugins, CDNs and reverse proxies can hide the original PHP fault or reintroduce its symptom with stale HTML. Purge the affected URL and test wp-login.php, wp-admin/, cart, checkout and account pages across the full redirect chain. Exclude dynamic and authenticated routes from full-page caching, and clear object caches when a plugin stores redirect state there. Inspect browser developer tools and the server access log; a cached 200 response where a 302 is expected points to the delivery layer rather than the header code itself.

Check the Hosting Environment

Server configuration can affect how the fault appears, but permanent output buffering is not a sound repair. Some cPanel or Plesk hosts enable output_buffering, while others disable it, which explains why the warning may show on one Australian server and remain hidden on another. Review the effective PHP configuration, PHP version and extension settings in the control panel. If the same request produces a warning in the PHP log, fix the ordering error; if it does not, treat the visible warning as a symptom of another problem.

File permissions do not normally cause this message, so avoid making everything writable as a first response. WordPress PHP files commonly use 644, directories commonly use 755, and ownership should match the account documented by the host; 777 adds avoidable risk. Verify core with WP-CLI’s wp core verify-checksums, compare plugin checksums, and check .htaccess for accidental blank lines or duplicated rules. Remove only duplicate or invalid directives—retain required rewrite, security and caching rules unless testing proves one is responsible.

Debug logs can reveal file paths, IP addresses and occasionally request data, so they should not sit in a publicly readable uploads directory or remain exposed indefinitely. For organisations covered by the Privacy Act 1988 and the Australian Privacy Principles, restrict access to diagnostic files, avoid collecting unnecessary customer details and apply the same retention controls used for other operational records. Production sites should record errors in authorised logs rather than display them to visitors. Rotating and deleting a temporary debug.log after the case is closed reduces both privacy exposure and the chance that stale output is mistaken for a current fault.

Prevent Recurrence and Protect Production

Make prevention part of the deployment routine. Keep custom PHP in version control, review changes on a staging copy, and run php -l or a WordPress coding-standard check before deployment. Test guest access to the login page, password resets, cart, checkout and restricted-page redirects because each path sends different headers. Australian teams often schedule updates after local trading hours; with Sydney and Brisbane sharing Eastern Time and Perth on Western Time, publish a maintenance window in AEST and confirm daylight-saving dates rather than relying on a vague “tonight” instruction.

Likely cause What to look for Corrective action
Stray output or UTF-8 BOM The warning names a specific file and line Remove whitespace or BOM and omit the closing PHP tag where appropriate
Plugin or theme sends output early The first log entry follows a plugin callback Update the extension, defer its output or correct the hook
Competing redirects A redirect loop or repeated Location attempts Keep one controlled redirect and terminate with exit
Cached or CDN response Behaviour changes after a cache purge Purge the response and exclude dynamic routes from full-page caching
Output buffering masks the fault The browser looks clean but PHP still logs an error Repair the original output order rather than relying on suppression
Changed or damaged files Core or plugin checksum verification fails Restore the affected files and correct ownership without using 777

Verification should be behavioural rather than visual. Confirm that a protected page responds with one clean 302, that login sets its cookies, and that a customer can move from cart to checkout without duplicate output. For Australian online retailers, that stability also supports compliance with the Australian Consumer Law, where consumer guarantees and clear representations matter; a broken checkout can complicate both customer service and evidence of how an offer was presented. Keep a dated record of the cause, correction and test results.

Once the first output is removed and redirects terminate correctly, the PHP warning should disappear from WordPress, PHP and server logs across ordinary and cached requests. Recheck after plugin updates, cache purges and PHP configuration changes, because the same early-output pattern can be reintroduced by a custom hook. A warning-free log, stable HTTP status sequence and successful checkout provide practical evidence that the WordPress installation is healthy.