PHP Headers Already Sent Error

14th February 2008 - 3 minutes read time

Note: This post is over a year old and so the information contained here might be out of date. If you do spot something please leave a comment and I will endeavour to correct.

Try running the following PHP script.

<?php
 echo 'browser output';
 session_start();
?>

You will either see normal output or get the following error messages.

This is because when you try to start the session it adds items to the headers outputted by the browser, including the setting up of cookies. To stop this happening you need to ensure that the session_start() function call is put before any output from the browser. This is the case for all header modifying functions including set_cookie() and header().

<?php
 session_start();
 echo 'browser output';
?>

These error messages can also be seen if you add any white space to your files before and after the and tags. To get around this you can use the trim() function on all of the files that you include. The following code will run through a directory and trim all of the white space from all of the files after backing the files up in case anything goes wrong.

<?php
$d = dir('folder') or die('Can\'t find directory');
while (false !== ($f = $d->read())) {
  $file = $d->path.'/'.$f;
  if (is_file($file)) {
    copy($file, $file.'.bak');
    $contents = trim(join('',file($file)));
    $fh = fopen($file,'w') or die('Can\'t open file: '.$file);
    if (-1 == fwrite($fh,$contents)) {
      die('Can\'t write to file: '.$file);
    }
    fclose($fh) or die('Can\'t close file: '.$file);
  }
}
?>

If you don't see the above error messages then your PHP engine has been set up with output buffering on. Open up the php.ini file and look for the following line.

output_buffering = 4096

Change it to this and restart the server.

output_buffering = off

Output buffering works by stopping all output to the browser until PHP has run through the code, at least up until the first 4096 bytes (the default option set in the php.ini file). It will then run any header scripts and the send any output to the browser. The drawback of using this method is that it can appear to slow down the server while the preprocessing is taking place as no output is displayed to screen until the last possible moment.

PHP

Add new comment

Related Content

Creating A Game With PHP Part 4: Side Scrolling Shooter

As another step up from the game of snake I created in my last post I decided to try my hand at creating a side scrolling shooter. Just like my other posts, this is an ASCII based game played on the command line.

A side scrolling shooter, if you didn't already know, moves a scene from right to left across the screen with enemies moving with the scene towards the player's ship, which is on the left hand side of the scene. The player can fire bullets towards the enemies so remove them from the scene.

Seeding Random Numbers in PHP

Computers are not that great at creating random numbers as the methods they use are deterministic. That is, they start from a number (called a seed) and apply maths to that number to generate pseudorandom numbers. Most random numbers generated by computers programs use a seed provided by the system, which is generally the current time. If you can guess the initial value then you can start to work out what the random sequence of numbers generated is. Most random numbers, are therefore not suitable for encryption.

PHP: Find An Array Sequence In An Array

I was looking for a function that searched an array for another array, and after not finding one I decided to write it. What I was looking for was a function that took a smaller array and searched for that exact sequence of items in a larger array. As it happens, PHP does have a number of array search functions, but they didn't produce the correct result.

PHP Live Regex

When creating regular expressions for PHP I tend to use the awesome tool PHP Live Regex.

This tool has really helped me over the last couple of years to create regular expressions for many of the projects I've been working on. Everything from validation functions to formatters that use regular expressions have had their expressions worked out using this tool. I even use it for testing one off expressions where I just need to find/replace in a file.

PHP Strict Types

In December 2015, PHP 7 introduced scalar type declarations and with it the strict_types flag. This flag can be used to enforce the type of value passed to functions. Although I have seen it in action in the past, I have never actually used it before so I decided to look into it a little to see what it was about.

PHP instanceof Operator Tips

The instanceof operator in PHP is great at making sure you are looking at a type of object before acting on it. Whilst it's straightforward to use on its own, using it can lead to some undesirable side effects.

As a basic example, let's create a couple of interfaces and classes to show the instanceof operator in action. Rather than just have some test names I am using object names that you might find in a system. This consists of User and Order that might form part of a commerce system.