Header Errors in PHP: How to Fix “Warning: Cannot Modify Header Information - Headers Already Sent”

🚫 Header Errors in PHP: Fix “Cannot Modify Header Information - Headers Already Sent”

If you’ve ever seen this warning in PHP:

Warning: Cannot modify header information - headers already sent by (output started at /path/to/file.php:xx)

Don’t panic! This is one of the most common PHP errors, especially for beginners working with header().

✅ What Does It Mean?

PHP is trying to send HTTP headers after some content (HTML, echo, or even a space) has already been sent to the browser.

🚀 How to Fix It Step by Step

1. Remove Output Before header()

Wrong:

echo "Welcome!";
header("Location: dashboard.php");

Right:

header("Location: dashboard.php");
exit();

2. No Output Before <?php

Wrong: (has space before opening tag)

 
<?php
header("Location: login.php");
?>

Right:

<?php
header("Location: login.php");
?>

3. Use Output Buffering

ob_start();
echo "Redirecting...";
header("Location: page.php");
exit();
ob_end_flush();

4. Remove UTF-8 BOM

Use editors like VS Code or Notepad++ and save your file as UTF-8 without BOM.

📌 Real-World Example

<?php
session_start();
if (!isset($_SESSION['logged_in'])) {
  header("Location: login.php");
  exit();
}
?>
<h1>Welcome!</h1>

⚠️ Precautions Table

Bad Practice	Good Practice
Echo before header()	Call header() first
Blank space before <?php	Start at line 1
Save with BOM	Use UTF-8 without BOM
Forget exit() after redirect	Always use exit()

❓ FAQs

Can I use header() after HTML?

No. Once output is sent, headers cannot be modified.

Can output buffering solve it?

Yes, but it's better to structure your code correctly.

Do I need exit() after header()?

Yes. Always use exit(); to prevent further code execution.

📈 SEO & AdSense Tips

• Fix PHP errors for better crawlability
• Use trending keywords like “php redirect error”, “headers already sent fix”
• Keep your page clean and mobile-friendly
• Use proper headings <h1>, <h2>, <h3>
• Add internal links to related PHP tutorials

🎉 Conclusion

Always send headers first, output later. If you follow the practices shared above, you’ll avoid this error and build cleaner, faster PHP applications.