{"id":5074,"date":"2026-06-25T14:28:39","date_gmt":"2026-06-25T14:28:39","guid":{"rendered":"https:\/\/www.copahost.com\/blog\/?p=5074"},"modified":"2026-06-25T14:33:13","modified_gmt":"2026-06-25T14:33:13","slug":"cannot-modify-header-information-headers-already-sent-by-how-to-fix-it","status":"publish","type":"post","link":"https:\/\/www.copahost.com\/blog\/cannot-modify-header-information-headers-already-sent-by-how-to-fix-it\/","title":{"rendered":"Cannot Modify Header Information \u2014 Headers Already Sent By: How to Fix It"},"content":{"rendered":"\n

The error “Warning: Cannot modify header information – headers already sent by” means a PHP script tried to send HTTP headers (for a redirect, cookie, or page setting) after content had already been sent to the browser. The usual cause is stray output \u2014 a blank space, line break, or text \u2014 before the headers, most often whitespace before <?php<\/code> or after ?><\/code> in a file. The good news: the error message tells you exactly which file and line to fix.<\/strong><\/p>\n\n\n\n

\"WordPress<\/figure>\n\n\n\n

If you’re seeing this warning across the top of your site \u2014 often after installing a plugin, editing your theme, or adding a redirect \u2014 don’t panic. It looks cryptic, but it’s one of the more logical WordPress errors to fix, because the message itself points you to the culprit. The trick is knowing how to read it, which is exactly what this guide will show you.<\/p>\n\n\n\n

What “headers already sent” means<\/h2>\n\n\n\n

When a browser requests a page, the server must send HTTP headers first<\/strong> (invisible instructions like content type, cookies, and redirects), and only then the content<\/strong> (the HTML you see). That order is fixed by the HTTP protocol.<\/p>\n\n\n\n

The “headers already sent” error happens when a PHP script tries to send a header \u2014 through a function like header()<\/code>, wp_redirect()<\/code>, setcookie()<\/code>, or session_start()<\/code> \u2014 after<\/strong> some content has already gone to the browser. Once output has started, it’s too late to send headers, so PHP throws the warning. You may also see it phrased simply as “headers already sent”<\/strong> or “cannot modify header information”<\/strong> \u2014 it’s the same error.<\/p>\n\n\n\n

This matters beyond the warning text itself: because the failed header is often a redirect, a cookie, or a login action, those features can silently stop working \u2014 broken redirects, users unable to log in, or sessions that won’t start \u2014 until the underlying output is removed.<\/p>\n\n\n\n

The “content” that slips out early is usually invisible: a stray space or blank line, a forgotten echo<\/code>, or hidden characters in a file. That’s why this error feels mysterious until you know where to look.<\/p>\n\n\n\n

The key skill: how to read the error message<\/h2>\n\n\n\n

This is what most people get wrong \u2014 and it’s the fastest way to fix the error. The message has three parts<\/strong>, and they don’t all mean the same thing:<\/p>\n\n\n\n

\n
\n Warning: Cannot modify header information - headers already sent by\n(output started at \/public_html\/wp-content\/themes\/mytheme\/functions.php:42)\nin \/public_html\/wp-includes\/pluggable.php on line 1450<\/code>\n <\/div>\n\n
\n
\n
1<\/div>\n
The error name.<\/strong> “Cannot modify header information \u2013 headers already sent by.” Tells you what kind of problem it is.<\/div>\n <\/div>\n
\n
2<\/div>\n
“output started at \u2026 :42” \u2014 THIS is the file to fix.<\/strong> The file and line where the stray output began. This is your culprit. Go here.<\/div>\n <\/div>\n
\n
3<\/div>\n
“in \u2026 pluggable.php on line 1450” \u2014 do NOT edit this.<\/strong> Usually a WordPress core file that simply failed because of part 2. It’s the victim, not the cause.<\/div>\n <\/div>\n <\/div>\n<\/div>\n\n\n\n

The crucial insight: the file in “output started at” is the one you fix<\/strong> \u2014 that’s where the stray output began. The file at the end (“in \u2026 on line \u2026”) is usually a WordPress core file like pluggable.php<\/code> that simply failed<\/em> because of the first file. Don’t edit that core file<\/strong> \u2014 it’s the victim, not the cause. Beginners often waste time editing the wrong file; reading the message correctly avoids that entirely.<\/p>\n\n\n\n

So your first step is always: read the message, note the file and line in the “output started at”<\/strong> part, and go there.<\/p>\n\n\n\n

Common causes<\/h2>\n\n\n\n
\n \n \n \n \n
Cause<\/th>\n What it looks like<\/th>\n <\/tr>\n <\/thead>\n
Whitespace (most common)<\/td>A space or blank line before <?php<\/code> or after ?><\/code>. Invisible, but counts as output.<\/td><\/tr>\n
Premature output<\/td>A stray echo<\/code>, print<\/code>, HTML, or leftover debug code before a header call.<\/td><\/tr>\n
Plugin or theme<\/td>Badly coded extension outputting early \u2014 often right after an update.<\/td><\/tr>\n
BOM (hidden bytes)<\/td>File saved as “UTF-8 with BOM” adds invisible characters before your code.<\/td><\/tr>\n <\/tbody>\n <\/table>\n<\/div>\n\n\n\n

By far the most common is whitespace<\/strong> \u2014 a space or blank line before the opening <?php<\/code> or after the closing ?><\/code> tag. It’s invisible in most editors but counts as output, so the headers fail. The second most common is a plugin or theme<\/strong> introducing premature output, especially right after an update.<\/p>\n\n\n\n

How to fix it<\/h2>\n\n\n\n

Step 1 \u2014 Open the file the message points to<\/h3>\n\n\n\n

Using your hosting File Manager<\/strong> or an FTP client<\/a>, open the file named in the “output started at”<\/strong> part of the message, and go to the line number it gives. (If your dashboard still works and the file is a theme or plugin file, you can sometimes use the built-in editor \u2014 but FTP is safer.)<\/p>\n\n\n\n

Always back up the file before editing it.<\/strong><\/p>\n\n\n\n

Step 2 \u2014 Remove the stray output<\/h3>\n\n\n\n

Look at the indicated line and the lines just before it. You’re hunting for output that shouldn’t be there:<\/p>\n\n\n\n