Developing websites and applications in PHP often involves working with HTTP headers, which facilitate communication between the server and the client. One common error developers encounter is the "Headers Already Sent" error. This error occurs when a script attempts to send an HTTP header after part of the response body has already been sent.
Causes of the Error
The "Headers Already Sent" error typically occurs for several reasons:
-
Whitespace before opening PHP tag or after closing PHP tag: Any output, including spaces or new lines before
<?phpor after?>, can cause PHP to start sending the response body before the headers. -
Echo, print, or other output functions: Using these functions before sending headers will cause the server to start sending the response body.
-
Errors and warnings: Any errors or warnings that generate output to the screen before sending headers can lead to this error.
Resolving the Error
To prevent the "Headers Already Sent" error, it's important to follow several best practices:
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Limit output before sending headers: Ensure that no output from the PHP script occurs before sending any headers.
-
Avoid closing PHP tag: In PHP files containing only PHP code, it's recommended to avoid the closing PHP tag (
?>) at the end of the file. This eliminates the risk of unintended whitespace after the closing tag. -
Output buffering: Using output buffering functions like
ob_start()allows you to capture output before it's sent to the client and handle it as a single block. This enables you to send headers at any point before flushing the buffer. -
Check files for whitespace: Review your PHP files, especially those included at the beginning of the script, for the presence of whitespace before the opening PHP tag or after the closing tag.
The "Headers Already Sent" error is often a sign of the need to revise and adjust how code is organized and how outputs are managed. With proper design and disciplined handling of PHP code, this error can be easily avoided, ensuring smooth operation of your applications.
