When running my script, I am getting several errors like this:
Warning: Cannot modify header information - headers already sent by (output started at /some/file.php:12) in /some/file.php on line 23
The lines mentioned in the error messages contain
What could be the reason for this? And how to fix it?
No output before sending headers!
Functions that send/modify HTTP headers must be invoked before any output is made. summary ? Otherwise the call fails:
Some functions modifying the HTTP header are:
Output can be:
echoand other functions producing output
Why does it happen?
To understand why headers must be sent before output it's necessary to look at a typical HTTP response. PHP scripts mainly generate HTML content, but also pass a set of HTTP/CGI headers to the webserver:
The page/output always follows the headers. PHP has to pass the headers to the webserver first. It can only do that once. After the double linebreak it can nevermore amend them.
When PHP receives the first output (
<html>) it will flush all collected headers. Afterwards it can send all the output it wants. But sending further HTTP headers is impossible then.
How can you find out where the premature output occured?
header()warning contains all relevant information to locate the problem cause:
Here "line 100" refers to the script where the
The "output started at" note within the parenthesis is more significant. It denominates the source of previous output. In this example it's
52. That's where you had to look for premature output.
Intentional output from
echostatements will terminate the opportunity to send HTTP headers. The application flow must be restructured to avoid that. Use functions and templating schemes. Ensure
header()calls occur before messages are written out.
Functions that produce output include
among others and user-defined functions.
Raw HTML areas
Unparsed HTML sections in a
.phpfile are direct output as well. Script conditions that will trigger a
header()call must be noted before any raw
Use a templating scheme to separate processing from output logic.
<?phpfor "script.php line 1" warnings
If the warning refers to output in line
1, then it's mostly leading whitespace, text or HTML before the opening
Similarly it can occur for appended scripts or script sections:
PHP actually eats up a single linebreak after close tags. But it won't compensate multiple newlines or tabs or spaces shifted into such gaps.
Linebreaks and spaces alone can be a problem. But there are also "invisible" character sequences which can cause this. Most famously the UTF-8 BOM (Byte-Order-Mark) which isn't displayed by most text editors. It's the byte sequence
EF BB BF, which is optional and redundant for UTF-8 encoded documents. PHP however has to treat it as raw output. It may show up as the characters
ï»¿in the output (if the client interprets the document as Latin-1) or similar "garbage".
In particular graphical editors and Java based IDEs are oblivious to its presence. They don't visualize it (obliged by the Unicode standard). Most programmer and console editors however do:
There it's easy to recognize the problem early on. Other editors may identify its presence in a file/settings menu (Notepad++ on Windows can identify and remedy the problem), Another option to inspect the BOMs presence is resorting to an hexeditor. On *nix systems
hexdumpis usually available, if not a graphical variant which simplifies auditing these and other issues:
An easy fix is to set the text editor to save files as "UTF-8 (no BOM)" or similar such nomenclature. Often newcomers otherwise resort to creating new files and just copy&pasting the previous code back in.
There are also automated tools to examine and rewrite text files (
recode). For PHP specifically there's the
phptagstag tidier. It rewrites close and open tags into long and short forms, but also easily fixes leading and trailing whitespace, Unicode and UTF-x BOM issues:
It's sane to use on a whole include or project directory.
If the error source is mentioned as behind the closing
?>then this is where some whitespace or raw text got written out. The PHP end marker does not terminate script executation at this point. Any text/space characters after it will be written out as page content still.
It's commonly advised, in particular to newcomers, that trailing
?>PHP close tags should be omitted. This eschews a small portion of these cases. (Quite commonly
include()dscripts are the culprit.)
Error source mentioned as "Unknown on line 0"
It's typically a PHP extension or php.ini setting if no error source is concretized.
gzipstream encoding setting or the
extension=module generating an implicit PHP startup/warning message.
Preceding error messages
If another PHP statement or expression causes a warning message or notice being printeded out, that also counts as premature output.
In this case you need to eschew the error, delay the statement execution, or suppress the message with e.g.
@()- when either doesn't obstruct debugging later on.
No error message
If you have
php.ini, then no warning will show up. But ignoring errors won't make the problem go away. Headers still can't be sent after premature output.
header("Location: ...")redirects silently fail it's very advisable to probe for warnings. Reenable them with two simple commands atop the invocation script:
set_error_handler("var_dump");if all else fails.
Speaking of redirect headers, you should often use an idiom like this for final code paths:
Preferrably even a utility function, which prints a user message in case of
Output buffering as workaround
PHPs output buffering is a workaround to alleviate this issue. It often works reliably, but shouldn't substitute for proper application structuring and separating output from control logic. Its actual purpose is minimizing chunked transfers to the webserver.
output_buffering=setting nevertheless can help. Configure it in the php.ini or via .htaccess or even .user.ini on modern FPM/FastCGI setups.
Enabling it will allow PHP to buffer output instead of passing it to the webserver instantly. PHP thus can aggregate HTTP headers.
It can likewise be engaged with a call to
ob_start();atop the invocation script. Which however is less reliable for multiple reasons:
<?php ob_start(); ?>starts the first script, whitespace or a BOM might get shuffled before, rendering it ineffective.
It can conceal whitespace for HTML output. But as soon as the application logic attempts to send binary content (a generated image for example), the buffered extraneous output becomes a problem. (Necessitating
ob_clean()as furher workaround.)
The buffer is limited in size, and can easily overrun when left to defaults. And that's not a rare occurence either, difficult to track down when it happens.
Both approaches therefore may become unreliable - in particular when switching between development setups and/or production servers. Which is why output buffering is widely considered just a crutch / strictly a workaround.
See also the basic usage example in the manual, and for more pros and cons:
But it worked on the other server!?
If you didn't get the headers warning before, then the output buffering php.ini setting has changed. It's likely unconfigured on the current/new server.
You can always use
headers_sent()to probe if it's still possible to... send headers. Which is useful to conditionally print an info or apply other fallback logic.
Useful fallback workarounds are:
If your application is structurally hard to fix, then an easy (but somewhat unprofessional) way to allow redirects is injecting a HTML
<meta>tag. A redirect can be achieved with:
Or with a short delay:
This leads to non-valid HTML when utilized past the
<head>section. Most browsers still accept it.
While this is often more HTML compliant than the
Both approaches however make acceptable fallbacks when genuine HTTP header() calls fail. Ideally you'd always combine this with a user-friendly message and clickable link as last resort. (Which for instance is what the http_redirect() PECL extension does.)
session_start()are also affected
session_start()need to send a
Set-Cookie:HTTP header. The same conditions therefore apply, and similar error messages will be generated for premature output situations.
(Of course they're furthermore affected by disabled cookies in the browser, or even proxy issues. The session functionality obviously also depends on free disk space and other php.ini settings, etc.)