How to fix Headers already sent error in PHP

Question

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 header   and setcookie   calls   What could be the reason for this  And how to fix it

User · Answer

It is because of this line   printf   Hi  s  lt  br   gt     name     You should not print echo anything before sending the headers

User · Answer

This error message gets triggered when anything is sent before you send HTTP headers  with setcookie or header   Common reasons for outputting something before the HTTP headers are    Accidental whitespace  often at the beginning or end of files  like this     lt  php    Note the space before   lt  php    gt      nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp To avoid this  simply leave out the closing   gt  - it s not required anyways    Byte order marks at the beginning of a php file  Examine your php files with a hex editor to find out whether that s the case  They should start with the bytes 3F 3C  You can safely remove the BOM EF BB BF from the start of files  Explicit output  such as calls to echo  printf  readfile  passthru  code before  lt   etc  A warning outputted by php  if the display errors php ini property is set  Instead of crashing on a programmer mistake  php silently fixes the error and emits a warning  While you can modify the display errors or error reporting configurations  you should rather fix the problem  Common reasons are accesses to undefined elements of an array  such as   POST  input   without using empty or isset to test whether the input is set   or using an undefined constant instead of a string literal  as in   POST input   note the missing quotes     Turning on output buffering should make the problem go away  all output after the call to ob start is buffered in memory until you release the buffer  e g  with ob end flush   However  while output buffering avoids the issues  you should really determine why your application outputs an HTTP body before the HTTP header  That d be like taking a phone call and discussing your day and the weather before telling the caller that he s got the wrong number

User · Answer

You do   printf   Hi  s  lt  br   gt     name     before setting the cookies  which isn t allowed  You can t send any output before the headers  not even a blank line

User · Answer

Sometimes when the dev process has both WIN work stations and LINUX systems (hosting) and in the code you do not see any output before the related line, it could be the formatting of the file and the lack of Unix LF (linefeed) line ending.

What we usually do in order to quickly fix this, is rename the file and on the LINUX system create a new file instead of the renamed one, and then copy the content into that. Many times this solve the issue as some of the files that were created in WIN once moved to the hosting cause this issue.

This fix is an easy fix for sites we manage by FTP and sometimes can save our new team members some time.

User · Answer

No output before sending headers   Functions that send modify HTTP headers must be invoked before any output is made  summary   Otherwise the call fails      Warning  Cannot modify header information - headers already sent  output started at script line    Some functions modifying the HTTP header are    header   header remove session start   session regenerate id setcookie   setrawcookie   Output can be    Unintentional    Whitespace before  lt  php or after   gt  The UTF-8 Byte Order Mark specifically Previous error messages or notices       Intentional    print  echo and other functions producing output Raw  lt html gt  sections prior  lt  php code      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   HTTP 1 1 200 OK Powered-By  PHP 5 3 7 Vary  Accept-Encoding Content-Type  text html  charset utf-8   lt html gt  lt head gt  lt title gt PHP page output page lt  title gt  lt  head gt   lt body gt  lt h1 gt Content lt  h1 gt   lt p gt Some more output follows    lt  p gt  and  lt a href     gt   lt img src internal-icon-delayed gt   lt  a gt    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  print  echo   lt html gt   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   The header   warning contains all relevant information to locate the problem cause      Warning  Cannot modify header information - headers already sent by    output started at  www usr2345 htdocs auth php 52  in    www usr2345 htdocs index php on line 100   Here  line 100  refers to the script where the header   invocation failed   The  output started at  note within the parenthesis is more significant  It denominates the source of previous output  In this example it s auth php and line 52  That s where you had to look for premature output   Typical causes    Print  echo  Intentional output from print and echo statements 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   print  echo  printf  vprintf trigger error  ob flush  ob end flush  var dump  print r readfile  passthru  flush  imagepng  imagejpeg    among others and user-defined functions  Raw HTML areas  Unparsed HTML sections in a  php file are direct output as well  Script conditions that will trigger a header   call must be noted before any raw  lt html gt  blocks    lt  DOCTYPE html gt   lt  php        Too late for headers already    Use a templating scheme to separate processing from output logic    Place form processing code atop scripts  Use temporary string variables to defer messages  The actual output logic and intermixed HTML output should follow last    Whitespace before  lt  php for  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  lt  php token     lt  php   There s a SINGLE space newline before  lt   - Which already seals it    Similarly it can occur for appended scripts or script sections     gt    lt  php   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  UTF-8 BOM  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        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 hexdump is 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 amp pasting the previous code back in   Correction utilities   There are also automated tools to examine and rewrite text files  sed awk or recode   For PHP specifically there s the phptags tag 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   phptags  --whitespace    php   It s sane to use on a whole include or project directory  Whitespace after   gt   If the error source is mentioned as behind the closing   gt  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   gt  PHP close tags should be omitted  This eschews a small portion of these cases   Quite commonly include  d scripts 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    It s occasionally the gzip stream encoding setting or the ob gzhandler  But it could also be any doubly loaded 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  isset   or     - when either doesn t obstruct debugging later on    No error message  If you have error reporting or display errors disabled per 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   So when header  Location        redirects silently fail it s very advisable to probe for warnings  Reenable them with two simple commands atop the invocation script   error reporting E ALL   ini set  display errors   1     Or 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   exit header  Location   finished html       Preferrably even a utility function  which prints a user message in case of header   failures   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    The 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    Even if  lt  php ob start      gt  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    What is output buffering  Why use output buffering in PHP  Is using output buffering considered a bad practice  Use case for output buffering as the correct solution to  quot headers already sent quot    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   Checking with headers sent    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   if  headers sent          die  Redirect failed  Please click on this link   lt a href     gt       else      exit header  Location   user php         Useful fallback workarounds are    HTML  lt meta gt  tag  If your application is structurally hard to fix  then an easy  but somewhat unprofessional  way to allow redirects is injecting a HTML  lt meta gt  tag  A redirect can be achieved with     lt meta http-equiv  Location  content  http   example com   gt    Or with a short delay     lt meta http-equiv  Refresh  content  2  url    target html  gt    This leads to non-valid HTML when utilized past the  lt head gt  section  Most browsers still accept it  JavaScript redirect  As alternative a JavaScript redirect can be used for page redirects     lt script gt  location replace  target html     lt  script gt    While this is often more HTML compliant than the  lt meta gt  workaround  it incurs a reliance on JavaScript-capable clients    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    Why setcookie   and session start   are also affected  Both setcookie   and 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    Further links   Google provides a lengthy list of similar discussions  And of course many specific cases have been covered on Stack Overflow as well  The Wordpress FAQ explains How do I solve the Headers already sent warning problem  in a generic manner  Adobe Community  PHP development  why redirects don t work  headers already sent  Nucleus FAQ  What does  page headers already sent  mean  One of the more thorough explanations is HTTP Headers and the PHP header   Function - A tutorial by NicholasSolutions  Internet Archive link   It covers HTTP in detail and gives a few guidelines for rewriting scripts

User · Answer

Another bad practice can invoke this problem which is not stated yet.

See this code snippet:

<?php
include('a_important_file.php'); //really really really bad practise
header("Location:A location");
?>

Things are okay,right?

What if "a_important_file.php" is this:

<?php
//some php code 
//another line of php code
//no line above is generating any output
?>

 ----------This is the end of the an_important_file-------------------

This will not work? Why?Because already a new line is generated.

Now,though this is not a common scenario what if you are using a MVC framework which loads a lots of file before handover things to your controller? This is not an uncommon scenario. Be prepare for this.

From PSR-2 2.2 :

All PHP files MUST use the Unix LF (linefeed) line ending.
All PHP files MUST end with a single blank line.
The closing ?> tag MUST be omitted from files containing only php

Believe me , following thse standards can save you a hell lot of hours from your life :)

User · Answer

Instead of the below line    header  Location   ADMIN URL   index php      write   echo   lt script gt location href      ADMIN URL   index php msg  msg   lt  script gt       or    gt  lt script gt  lt  php echo  location href      ADMIN URL   index php msg  msg       gt  lt  script gt  lt  php   It ll definitely solve your problem  I faced the same problem but I solved through writing header location in the above way

User · Answer

A simple tip  A simple space  or invisible special char  in your script  right before the very first  lt  php tag  can cause this   Especially when you are working in a team and somebody is using a  weak  IDE or has messed around in the files with strange text editors   I have seen these things

User · Answer

Generally this error arise when we send header after echoing or printing. If this error arise on a specific page then make sure that page is not echoing anything before calling to start_session().

Example of Unpredictable Error:

 <?php //a white-space before <?php also send for output and arise error
session_start();
session_regenerate_id();

//your page content

One more example:

<?php
includes 'functions.php';
?> <!-- This new line will also arise error -->
<?php
session_start();
session_regenerate_id();

//your page content

Conclusion: Do not output any character before calling session_start() or header() functions not even a white-space or new-line

User · Answer

COMMON PROBLEMS    copied from  source                         1  there should not be any output  i e  echo   or HTML codes  before the header           command   2  remove any white-space or newline  before  lt  php and after   gt  tags   3   GOLDEN RULE  - check if that php file  and also  if you include other files  have UTF8 without BOM encoding  and not just UTF-8   That is problem in many cases  because UTF8 encoded file has something special character in the start of php file  which your text-editor doesnt show               4  After header       you must use exit     5   always use 301 or 302 reference   header  location  http   example com    true   301     exit    6  Turn on error reporting  and find the error   Your error may be caused by a function that is not working  When you turn on error reporting  you should always fix top-most error first  For example  it might be  Warning  date default timezone get    It is not safe to rely on the system s timezone settings   - then farther on down you may see  headers not sent  error  After fixing top-most  1st  error  re-load your page  If you still have errors  then again fix the top-most error   7  If none of above helps  use JAVSCRIPT redirection however  strongly non-recommended method   may be the last chance in custom cases       echo   lt script type  text javascript  gt window top location  http   website com    lt  script gt    exit

User · Answer

I got this error many times before  and I am certain all PHP programmer got this error at least once before   Possible Solution 1  This error may have been caused by the blank spaces before the start of the file or after the end of the file These blank spaces should not be here   ex      THERE SHOULD BE NO BLANK SPACES HERE          echo  your code here      gt  THERE SHOULD BE NO BLANK SPACES HERE   Check all files associated with file that causes this error    Note  Sometimes EDITOR IDE  like gedit  a default linux editor  add one blank line on save file  This should not happen  If you are using Linux  you can use VI editor to remove space lines after    at the end of the page   Possible Solution 2  If this is not your case  then use ob start to output buffering    lt  php   ob start          code    ob end flush      gt     This will turn output buffering on and your headers will be created after the page is buffered

[php] How to fix "Headers already sent" error in PHP

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COMMON PROBLEMS:

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