The error message "Warning: session_start(): Cannot start session when headers already sent" is a common warning that developers often encounter when working with PHP, especially when attempting to start a new session or resume an existing session after HTTP headers have been sent. This warning indicates that your script attempted to start a session after HTTP headers have already been sent to the browser. In PHP, once headers are sent, certain operations like starting a session or setting cookies cannot be performed because these details must be part of the headers.
Root Causes
1. Output before session_start() call
One of the most common causes of this error is output (echo, print, or white spaces outside PHP tags) being generated before the session_start() call. In PHP scripts, no output should precede this function call.
2. BOM (Byte Order Mark)
Another potential issue could be the presence of a Byte Order Mark (BOM) at the beginning of a PHP file. The BOM is a hidden sequence of bytes that can be used to identify the file encoding. Unfortunately, if present in a PHP file, it can be interpreted as output.
3. Automatic header sending
In some instances, headers may be automatically sent by the server or PHP configuration (for example, when output_buffering is used).
Solutions
1. Verify session_start() placement
Ensure that the session_start() call is placed before any output in the script. This includes HTML tags, echo, print statements, or even white spaces outside PHP tags.
2. Check and remove BOM
If the problem is caused by a BOM, use a text editor with support for displaying and removing BOMs to ensure your files are saved without a BOM.
3. Proper output_buffering configuration
If possible, you can also use the output_buffering directive in the php.ini file or via the ini_set() function in your script to send all headers at once, which can prevent unwanted header sending.
4. Use ob_start()
Another solution could be to use ob_start() at the beginning of your script, which initiates output buffering. This prevents any headers or output from being sent until you explicitly call ob_end_flush() or a similar function.
The "Warning: session_start(): Cannot start session when headers already sent" error is often caused by fundamental mistakes in handling script output. By understanding and correctly applying the solutions mentioned above, you can effectively resolve this issue and prevent its occurrence in the future.
