WordPress 'Cannot Modify Header Information - Headers Already Sent' Fix (2026)

"Warning: Cannot modify header information - headers already sent by (output started at /home/user/.../wp-config.php:1) in /wp-includes/pluggable.php on line 14xx." Pluggable.php is the victim, not the culprit. The real culprit is in the parentheses: one invisible character of output somewhere. Here's how to find and kill it.

Step 1: Read the Right Part of the Warning

Ignore "pluggable.php on line X" - that's where WordPress tried to send a header. What matters: output started at [file]:[line]. That file and line is where the stray output lives. wp-config.php line 1 and functions.php are responsible for most cases.

Step 2: Fix wp-config.php (The Usual Suspect)

1. 1.Open wp-config.php via FTP in a real code editor (VS Code, Notepad++ - not Windows Notepad)
2. 2.Check character position 1,1: the file must start with the PHP open tag - delete any space, blank line, or invisible character before it
3. 3.Scroll to the end: if there's a closing PHP tag, delete it and any blank lines after - the closing tag is optional and trailing whitespace after it is a classic trigger
4. 4.Save as UTF-8 without BOM (in Notepad++: Encoding menu; in VS Code: click encoding in status bar). The BOM is 3 invisible bytes of output at line 1

Step 3: Check functions.php and Recently Edited Files

If wp-config is clean, apply the same three checks to the file named in the warning - usually the active theme's functions.php or a file you edited just before the warning appeared: nothing before the open tag, no closing tag at EOF, UTF-8 no BOM. If the named file is inside wp-content/plugins/[name]/, that's a plugin bug - update or replace that plugin instead of editing it.

Step 4: Find Echo Statements Before Redirects

Custom code with echo, print, or var_dump running in functions.php or a plugin before WordPress sends headers causes the same warning - and the "output started at" line will point exactly at it. Remove the debug output or move it inside a hook that runs after headers.

Why You Shouldn't Just Hide the Warning

Turning off display_errors hides the symptom but headers still fail silently: logins loop, redirects break, contact forms submit to blank pages. Fix the whitespace. Related session symptom: wp-admin login redirect loop. If editing files broke something else: syntax error fix.