Warning: Cannot modify header information - headers already sent
Happens when your script tries to send an HTTP header to the client but there already was output before, which resulted in headers to be already sent to the client.
This is an
E_WARNING and it will not stop the script.
A typical example would be a template file like this:
<html>
<?php session_start(); ?>
<head><title>My Page</title>
</html>
...
The
session_start() function will try to send headers with the session cookie to the client. But PHP already sent headers when it wrote the <html> element to the output stream. You'd have to move the session_start() to the top.
You can solve this by going through the lines before the code triggering the Warning and check where it outputs. Move any header sending code before that code.
An often overlooked output is new lines after PHP's closing
?>. It is considered a standard practice to omit ?> when it is the last thing in the file. Likewise, another common cause for this warning is when the opening <?php has an empty space, line, or invisible character before it, causing the web server to send the headers and the whitespace/newline thus when PHP starts parsing won't be able to submit any header.
If your file has more than one
<?php ... ?> code block in it, you should not have any spaces in between them. (Note: You might have multiple blocks if you had code that was automatically constructed)
Also make sure you don't have any Byte Order Marks in your code, for example when the encoding of the script is UTF-8 with BOM.
Related Questions:
- Headers already sent by PHP
- All PHP "Headers already sent" Questions on Stackoverflow
- Byte Order Mark
- What PHP Functions Create Output?
Fatal error: Call to a member function ... on a non-object
Happens with code similar to
xyz->method() where xyz is not an object and therefore that method can not be called.
This is a fatal error which will stop the script (forward compatibility notice: It will become a catchable error starting with PHP 7).
Most often this is a sign that the code has missing checks for error conditions. Validate that an object is actually an object before calling its methods.
A typical example would be
// ... some code using PDO
$statement = $pdo->prepare('invalid query', ...);
$statement->execute(...);
In the example above, the query cannot be prepared and
prepare() will assign false to $statement. Trying to call the execute() method will then result in the Fatal Error because false is a "non-object" because the value is a boolean.
Figure out why your function returned a boolean instead of an object. For example, check the
$pdo object for the last error that occurred. Details on how to debug this will depend on how errors are handled for the particular function/object/class in question.
If even the
->prepare is failing then your $pdo database handle object didn't get passed into the current scope. Find where it got defined. Then pass it as a parameter, store it as property, or share it via the global scope.
Another problem may be conditionally creating an object and then trying to call a method outside that conditional block. For example
if ($someCondition) {
$myObj = new MyObj();
}
// ...
$myObj->someMethod();
Nothing is seen. The page is empty and white.
Also known as the White Page Of Death or White Screen Of Death. This happens when error reporting is turned off and a fatal error (often syntax error) occurred.
If you have error logging enabled, you will find the concrete error message in your error log. This will usually be in a file called "php_errors.log", either in a central location (e.g. /var/log/apache2 on many Linux environments) or in the directory of the script itself (sometimes used in a shared hosting environment).
Sometimes it might be more straightforward to temporarily enable the display of errors. The white page will then display the error message. Take care because these errors are visible to everybody visiting the website.
This can be easily done by adding at the top of the script the following PHP code:
ini_set('display_errors', 1); error_reporting(~0);
The code will turn on the display of errors and set reporting to the highest level.
Since the ini_set() is executed at runtime it has no effects on parsing/syntax errors. Those errors will appear in the log. If you want to display them in the output as well (e.g. in a browser) you have to set the display_startup_errors directive to true. Do this either in the php.ini or in a .htaccess or by any other method that affects the configuration before runtime.
You can use the same methods to set the log_errors and error_log directives to choose your own log file location.
Looking in the log or using the display, you will get a much better error message and the line of code where your script comes to halt.
Related questions:
- PHP's white screen of death
- White screen of death!
- PHP Does Not Display Error Messages
- PHP emitting 500 on errors - where is this documented?
- How to get useful error messages in PHP?
- All PHP "White Page of Death" Questions on Stackoverflow
Related errors:
- Parse error: syntax error, unexpected T_XXX
- Fatal error: Call to a member function ... on a non-object
- Code doesn't run/what looks like parts of my PHP code are output
Notice: Undefined Index
Happens when you try to access an array by a key that does not exist in the array.
A typical example of an Undefined Index notice would be (demo)
$data = array('foo' => '42', 'bar');
echo $data['spinach'];
echo $data[1];
Both spinach and 1 do not exist in the array, causing an E_NOTICE to be triggered.
The solution is to make sure the index or offset exists prior to accessing that index. This may mean that you need to fix a bug in your program to ensure that those indexes do exist when you expect them to. Or it may mean that you need to test whether the indexes exist using array_key_exists or isset:
$data = array('foo' => '42', 'bar');
if (array_key_exists('spinach', $data)) {
echo $data['spinach'];
}
else {
echo 'No key spinach in the array';
}
If you have code like:
<?php echo $_POST['message']; ?>
<form method="post" action="">
<input type="text" name="message">
...
then $_POST['message'] will not be set when this page is first loaded and you will get the above error. Only when the form is submitted and this code is run a second time will the array index exist. You typically check for this with:
if ($_POST) .. // if the $_POST array is not empty
// or
if ($_SERVER['REQUEST_METHOD'] == 'POST') .. // page was requested with POST
