How to Fix the “Cannot Modify Header Information – Headers Already Sent By” Error


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How to Fix the 'cannot modify header information - headers already sent by' error

Despite the best intentions of programmers, regardless of their experience level, errors are common occurrences in coding and website development. One common error that leaves many web devs their heads is the common WordPress warning: cannot modify header information – headers already sent by PHP, which is a  problem in WordPress’ PHP script.

Facing this error can be very frustrating, especially if you don’t know its root cause and how to troubleshoot it effectively. This error can disrupt your website’s seamless function, degrade the end-user experience, and even negatively impact your SEO rankings if left unattended—but don’t worry; we’re here to help you resolve this fairly common error. 

We’ll review some of its most common causes and present you with a systematic, step-by-step approach to help you finally resolve it and get your site back up and running again. But, before we jump into the specifics, please note that it’s crucial to use a robust backup plugin, like BlogVault, to help secure your data before making any changes to your site—after all, it’s always better to be safe than sorry.

TL;DR: This guide covers the most common causes of the “cannot modify header information – headers already sent by” error and offers step-by-step solutions to fix it. For your data integrity and safety, remember to use a robust backup plugin like BlogVault before making any changes to your site.

What Triggers The “Cannot Modify Header Information – Headers Already Sent By” Error?

Cannot modify header information – headers already sent by the warning is a common problem with PHP scripts that developers often encounter. It’s triggered when a PHP script attempts to send HTTP headers after some output has already been sent to the browser. 

 cannot modify header information - headers already sent by warning


But what does this mean? And what’s causing this error to happen? Let’s explore some of the more common culprits behind this pesky problem.

Understanding HTTP Headers and Their Role in Web Dev

Before going over the causes of this error, you need to understand what HTTP headers are and their overall role in web development. HTTP headers are an integral part of any HTTP response or request, acting as the metadata carrier for the HTTP message. 

These headers handle tasks like providing information about the server or client, controlling the cache settings, managing cookies, redirecting users, and so on. These headers must be sent before the actual content of the page (HTML, CSS, JavaScript, etc.). PHP, being a server-side scripting language, allows developers to modify these headers using functions like `header()`.

However, PHP comes with a condition – the `header()` function must be called before any actual output (like HTML) is sent to the browser. If any output has been sent before calling the `header()` function, PHP fails to send the HTTP headers, resulting in the PHP “cannot modify header information – headers already sent by” error. 

Now that we have a grasp on HTTP headers and what they do, let’s look at some of the common causes of the error:

Whitespace Before PHP Opening Tag or After PHP Closing Tag

In a PHP file, nothing should come before the opening `<?php` tag or after the closing `?>` tag. This includes any characters or whitespace. PHP interprets anything outside its opening and closing tags as output to be sent to the browser. If any whitespace or other characters exist, they cause output to start, preventing PHP from sending headers afterward. 

To avoid this, ensure your PHP files begin with `<?php` without any leading characters or whitespace and end with `?>` without any trailing characters or whitespace.

HTML Output Before header() Function Call

If you have any echo statements, print statements, or HTML outside PHP tags before the `header()` function call, this will send output to the browser, thereby causing the PHP warning “cannot modify header information – headers already sent by”. Therefore, ensuring that the `header()` function call is made before any output is sent is crucial to avoid errors. 

UTF-8 Byte Order Mark (BOM)

Byte Order Mark, or BOM, is a Unicode character used to signify a text file or stream’s endianness (byte order). Some text editors, especially those that support UTF-8 encoding, add a BOM at the beginning of files. This BOM, although invisible, is considered output, which can ultimately trigger the error. 

Issues With Output Buffering

Output buffering is a mechanism in PHP where the system stores output data into a buffer before it’s sent to the browser. If the output buffering settings are misconfigured or turned off, PHP sends output immediately instead of storing it in the buffer, which can cause the error in question. 

How to Fix “Cannot Modify Header Information – Headers Already Sent By” Error

Dealing with this error doesn’t have to be complicated—the key to resolving it is to approach the issue systematically, understanding what’s causing the error in the first place. 

To make the process a little easier, here’s a detailed step-by-step guide to help you effectively resolve this common error:

Step 1: Backup Your Website Before Beginning 

Before you proceed with any troubleshooting, it’s essential to have a full backup of your website. This ensures you can quickly restore your website to its previous state if something goes wrong during the debugging process. To help, you can use a backup plugin like BlogVault or rely on the backup services provided by your WordPress hosting provider to help.


Step 2: Identifying the Problematic File

The error message usually provides the path and the name of the file causing the problem. The file might be a theme, plugin, or even a core WordPress file. The error also shows the line number where the output started. This information is crucial to pinpoint the cause of the error.

Step 3: Check for Whitespace Before PHP Opening Tag or After PHP Closing Tag

Access the problematic file via an FTP client or your hosting File Manager. Check for any whitespace or characters before the PHP opening tag <?php or after the PHP closing tag ?>. If there is, delete it. Ensure your PHP file starts with <?php and ends with ?> without any extra characters or whitespace.

Step 4: Check for HTML Output Before header() Function Call

Examine your PHP file for any echo statements, print statements, or HTML outside PHP tags before the header() function call. These lines of code should be moved after the header() function call or within output buffering statements.

Step 5: Check for UTF-8 Byte Order Mark (BOM)

You should also check if your file has a UTF-8 BOM at the beginning, which can be done using a text editor that supports UTF-8 encoding, like Notepad++. If a BOM is present in your code, ensure it’s removed before proceeding.

Step 6: Check and Correct PHP Output Buffering Settings

In your PHP configuration file (php.ini), look for the output buffering setting—if it’s turned off or misconfigured, it may cause the error. In general, you should set output_buffering = On to enable output buffering. If you don’t have access to the php.ini file, you can turn on output buffering in your script by calling the ob_start() function at the beginning of your script.

Step 7: Thoroughly Test Your Website

Once you’ve made these changes, save your file, upload it back to your server, and refresh your website. If the error was due to one of the above issues, it should now be resolved. If it’s not, you may need to repeat the process with any other files indicated in the error message.

Step 8: Reach Out to Professionals If Needed

If you’ve gone through the above steps and the error persists, it might be time to contact a professional developer or your hosting provider’s support team. They can help investigate the issue further and provide a solution.

Remember, dealing with PHP and server configuration issues can be tricky and may result in additional errors and problems, so there’s no harm in asking for professional help when you think you need it.

Carefully following this step-by-step guide can help you resolve this common error and get your site back up and running in no time. As with any technical issue, understanding the problem and approaching it carefully is key to resolving it effectively.

Frequently Asked Questions

Can a plugin conflict cause the “Cannot modify header information – headers already sent by” error in WordPress?

Yes, a plugin conflict can cause this common conflict in WordPress. Because plugins ultimately add new functionality to your website, conflicts can arise due to their core code or even potential disagreements with other plugins or WordPress’s core.

When this error pops up, it’s typically because a plugin has sent output prematurely before HTTP headers have been fully processed. This output could be as simple as an extra space or line break in the PHP file. If you’ve recently installed a plugin and encountered this error, a plugin conflict could likely be the root cause.

You can diagnose this by deactivating all your plugins temporarily. If the error vanishes, it confirms that a plugin is at fault. You can then reactivate each plugin individually, refreshing your site after each one—whichever plugin reintroduces the error is causing the problem.

In this situation, you have several courses of action. For example, you could contact the plugin’s developer or support team with details about the error, as they can provide a direct solution or even fix the bug in a subsequent update. 

Alternatively, if the problematic plugin isn’t promptly resolved, you might consider replacing it with another plugin that offers similar features but doesn’t trigger the error. If the plugin is critical for your site and no suitable substitute exists, hiring a professional developer to amend the plugin’s underlying code could be a viable option worth considering.

Remember to always back up your site before adjusting plugins or code. A recent backup protects you from potential missteps by restoring your site to its prior state if necessary. This careful approach will help you efficiently handle the error when it’s instigated by a plugin conflict.

What is the impact of the “Cannot modify header information – headers already sent by” error?

This error is far more than just an inconvenience; it can profoundly impact your website as a whole and its overall performance. This error is a common issue in PHP development, and when it occurs, it can significantly disrupt your website’s functioning and your users’ experience.

First and most importantly, this error can interfere with browsers properly displaying and rendering your website. Since headers are responsible for sending vital information to the browser about handling your site’s data, any errors in the header modification process can result in faulty or incomplete page rendering. 

This means that visitors might encounter blank pages, improperly formatted content, or incomplete features, which can all lead to a poor user experience, causing visitors to leave your site prematurely, which, in turn, can increase your website’s bounce rate.

This error can also interfere with specific functionalities of your site, especially those relying on sessions or cookies. For instance, it can affect user authentication processes, shopping cart functionalities, and other features that depend on setting or modifying HTTP headers.

Beyond the core user experience, this error can also impact your website’s SEO ranking as well. Search engines consider site performance and user experience as key factors in determining overall page rankings. Therefore, recurring errors and high bounce rates can lead to lower rankings in search engine results, decreasing your site’s visibility and lowering organic traffic.

Finally, the constant occurrence of this error can also indicate deeper issues with your site’s coding or configuration. Therefore, addressing this error promptly is essential to maintain your website’s overall health and performance in the long term.

How can we avoid the “Cannot modify header information – headers already sent by” error in the future?

Avoiding this error in the future calls for mindful coding practices, which encompass smart programming habits, careful plugin management, and a good understanding of PHP output buffering.

  • Good Coding Practices: Ensuring proper PHP coding habits can go a long way in preventing this error. Therefore, it’s essential to avoid any output (like echo statements, HTML, or whitespace) before the HTTP headers are fully sent to the browser. You should avoid closing PHP tags at the end of files, as this can inadvertently create extra whitespace. Also, always use a UTF-8 encoded text editor without BOM for creating or editing PHP files.
  • Plugin Management: Be cautious when installing new plugins. Plugins can introduce unexpected output and cause conflicts. Always research and choose reputable plugins, keep them up to date, and regularly audit your plugins for any potential issues. When updating or adding a new plugin, it’s wise to test it in a staging environment first before implementing it on your live site.
  • Understanding PHP Output Buffering: PHP output buffering can prevent headers already sent errors by storing all output data in a buffer until the script finishes executing or the buffer is manually flushed. Enabling output buffering (output_buffering = On in the php.ini file) can help prevent PHP from sending output immediately, thus avoiding header modification errors.
  • Regular Site Maintenance and Monitoring: Regularly monitoring your site for errors and maintaining up-to-date systems can help you spot and rectify potential issues before they escalate.

Preventing this error requires diligent coding habits, careful plugin management, and regular site maintenance. Thankfully, you can keep this frustrating error at bay and ensure a smoother, more reliable experience for your website visitors after you adopt these best practices into your workflow.

What does the “Cannot modify header information – headers already sent by” error message mean?

Understanding the meaning behind this error can help you effectively resolve it and even avoid such issues again in the future. This error message primarily relates to how your website’s PHP scripts handle output and HTTP headers and the problems that can arise when these processes aren’t aligned correctly.

The HTTP headers of a website are crucial components that help browsers interpret and display your website correctly. They send necessary metadata about your webpage, such as the content type, character set, cache directives, etc. Ideally, these headers should be completely defined and sent to the browser before any other output from your PHP scripts.

The “Cannot modify header information – headers already sent by” error message typically indicates that something in your PHP code has caused output to be sent before the HTTP headers are fully defined. This can be caused by several factors, such as PHP echo statements, HTML code, or even white space at the beginning or end of your PHP files.

This error message also often includes specific file names and line numbers to indicate where the premature output originated. This detailed information can serve as a valuable lead in your debugging process. However, the source of the issue may not always be limited to the provided file or line. The problem can sometimes stem from an included or required file or a plugin or theme conflict.

Ultimately, the error message signifies a premature output sent before the HTTP headers, disrupting the sequential order of your website’s data processing. Understanding what this message implies can help you pinpoint the causes and devise a proper course of action to address and rectify the issue, helping ensure your site runs seamlessly.


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