Whitespace before output-causes and fixes

Whitespace output refers to invisible spaces, tabs, newlines, or the UTF-8 BOM before your PHP code runs. PHP treats this as output, which causes "Headers already sent" errors or breaks functionality.

Common causes

• UTF-8 BOM: File saved with BOM by Windows editors (see header-already-sent error)
• Whitespace after ?> at the end of files
• Blank lines before <?php tag
• HTML before PHP code: Content output before headers
• Multiple files included: One file has trailing whitespace that pollutes the output

How whitespace output appears

• "Headers already sent" error when trying to redirect or set cookies
• Extra blank line at the top of the page (visible in View Source)
• JSON output breaks because of extra whitespace before the {
• WordPress installation fails with "headers already sent" message

How to fix whitespace output

Step 1: Check for UTF-8 BOM

This is the most common cause. Open the affected file in your editor:

• VS Code: File → Save with Encoding → UTF-8 without BOM
• Notepad++: Encoding → Encode in UTF-8 without BOM
• Sublime Text: View → Line Endings (ensure it shows UTF-8, not UTF-8 with BOM)

Step 2: Remove trailing whitespace

Check files (especially wp-config.php, functions.php, and custom plugins):

<?php
// Your code here
// Remove any blank lines below this closing tag
?>

Better yet, omit the closing ?> tag if the file contains only PHP:

<?php
// Your code here
// No closing ?> needed

Step 3: Check for HTML before PHP

Ensure no HTML, spaces, or newlines appear before the opening <?php tag:

<?php
// This should be the very first line
// No whitespace above this tag
?>

Prevention

• Always save PHP files as UTF-8 without BOM
• Omit closing ?> tag in pure PHP files (no whitespace after it)
• Use a good code editor (VS Code, Sublime, PhpStorm) that highlights whitespace issues
• Configure your editor to trim trailing whitespace automatically
• Use PHP-FPM strict mode or linting tools to catch these before production

Related: Headers already sent | Parse errors

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