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What are common causes of headers already sent errors in PHP and how can I prevent them?

Hey everyone,

I'm fairly new to PHP and I've been encountering an issue with "headers already sent" errors in my code. I'm not quite sure what is causing this error and how to prevent it, so I was hoping someone here could shed some light on this issue.

From what I understand, the "headers already sent" error occurs when PHP tries to send headers (like cookies or HTTP responses) to the browser, but it has already sent some output before that, such as HTML, whitespace, or any sort of content.

I've done some research and found a few potential causes for this error, but I'm not sure which one is causing the problem in my specific case. Some common causes I came across include:

1. Whitespace or blank lines before the opening `<?php` tag in a file.
2. Including files that have whitespace or output before the `include` or `require` statement.
3. Outputting content before calling the `header()` function.
4. Using functions like `echo`, `print`, or `var_dump` before sending headers.
5. UTF-8 encoding issues or byte order marks (BOMs) at the beginning of files.

Are there any other potential causes I should be aware of? And most importantly, how can I prevent or fix these "headers already sent" errors? I really want to understand this issue better, so any insights or explanations would be greatly appreciated.

Thanks in advance!

All Replies


Hey there!

I've also stumbled upon the notorious "headers already sent" problem while working with PHP, and I can understand the frustration it brings. One aspect that has caused me headaches in the past is the presence of UTF-8 BOM (byte order mark) characters sneaking into my files.

Sometimes, when editing files with certain text editors or IDEs, they automatically insert the UTF-8 BOM at the beginning of the file. This seemingly insignificant addition can wreak havoc and trigger the "headers already sent" error. Whenever I encountered this issue, meticulously removing the BOM characters from my PHP files resolved the problem entirely.

Another thing to look out for is the usage of output buffering functions in your code. By employing functions like `ob_start()` at the beginning of your script or utilizing output buffering in relevant sections, you can capture any output and prevent it from being sent prematurely. This way, you ensure that headers are sent only when everything is ready, eliminating those pesky errors.

Additionally, keep an eye on any include files that might inadvertently generate output. For example, if you include a file that contains an errant `echo` statement, it will send output before headers, resulting in the error. Review your included files and diligently check for any unintended output-generating code.

Sometimes, this issue can also arise due to poorly placed PHP closing tags `?>` at the end of files. If there is any whitespace or unintended characters after the closing tag, they will be sent to the output buffer and can cause the "headers already sent" error. Ensure that you remove any unnecessary whitespace or avoid using closing PHP tags altogether at the end of pure PHP files to avoid this hassle.

By paying attention to these potential causes and implementing best practices like removing BOM characters, utilizing output buffering, reviewing included files, and handling closing tags wisely, I managed to keep those "headers already sent" errors at bay in my PHP projects.

Wishing you success in resolving this issue!

Best regards!


Hey folks,

I've also encountered the dreaded "headers already sent" problem in PHP, and it can be quite a headache to deal with. In my experience, one particular cause that tends to slip under the radar is the usage of whitespace or BOM (byte order mark) characters in included files.

It's essential to double-check not only for whitespace after the closing `?>` tag in included files but also within the files themselves. Even a single accidental space or line break before the opening `<?php` tag can wreak havoc and trigger the "headers already sent" error. Removing any unnecessary whitespace throughout your codebase might spare you from this troublesome issue.

Another sneaky culprit can be problematic file encoding. In some cases, when files are saved with certain encodings or include a BOM, it can lead to premature output being sent, causing the error. I'd suggest verifying that all your PHP files are consistently saved in UTF-8 encoding without any BOM characters. This small precaution saved me from countless hours of debugging.

Additionally, keep an eye out for any output generated by PHP errors or notices. These messages might not only disrupt your desired execution flow but also send unexpected output before you intended to send headers. Review any error_reporting-related configurations and ensure that error messages are redirected to logs instead of being displayed in the browser.

Lastly, if you're using frameworks or CMS platforms, they might have certain mechanisms that trigger output before headers are sent. It's worth exploring the documentation or relevant forums specific to the framework or CMS you're using to see if others have encountered similar issues and identify any recommended workarounds or solutions they suggest.

By diligently resolving issues related to whitespace, encoding, error output, and framework-specific quirks, I managed to mitigate "headers already sent" errors significantly in my projects. Remember to conduct thorough checks and pinpoint any potential sources of unintended output before headers are sent.

Wishing you the best of luck in resolving this issue!



Hey there,

I’ve faced this "headers already sent" issue in my PHP projects before, and it can be quite frustrating. Based on my experience, one common cause that often gets overlooked is the presence of whitespace or blank lines after the closing `?>` tag in included files.

Sometimes we tend to include files that have unnecessary whitespace at the end, which gets sent to the output buffer, causing the "headers already sent" error. Make sure to check for extra lines or spaces after the closing PHP tag in all your included files.

Another thing to be cautious of is any output generated by error messages or warning notices. If you have warnings or errors enabled in your development environment, they may output information to the browser, thus triggering the same error. In such cases, I found it helpful to check the error logs and fix any issues that may be generating output unintentionally.

Furthermore, if you're using an IDE or text editor with automatic file encoding, ensure that your PHP files are saved with consistent encoding, preferably UTF-8 without a byte order mark (BOM). Sometimes, different encodings or a BOM at the beginning of the file can lead to the "headers already sent" error.

Lastly, make sure to inspect your code for any instances where content is being output before the `header()` function is called. Double-check any code that uses functions like `echo`, `print`, or `var_dump` to ensure they aren't being executed before setting headers.

By taking these precautions, I managed to resolve many instances of "headers already sent" errors in my projects. Remember to carefully review your code and ensure there is no unintended output before headers are sent. I hope these suggestions help you overcome this issue too.

Good luck!

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