Why Am I Getting the PHP Warning: 'Cannot modify header information - headers already sent'?

PHP Error Reference: "Warning: Cannot modify header information - headers already sent"

This error occurs when PHP attempts to send HTTP headers to the client, but output has already been sent. As a warning (E_WARNING), it does not terminate script execution.

Causes:

The typical cause is output being generated before PHP executes code that sends headers, such as:

echo "Hello World";
header("Location: https://example.com");

In this example, echo sends output before the header function can set the "Location" header.

Solution:

Identify any code that generates output before headers are sent and move it after the header-sending code.

Common Causes:

• Empty spaces, newlines, or invisible characters before the opening
• Multiple code blocks with spaces or newlines in between
• Byte Order Marks (BOMs) in the code
• Output functions called before headers are sent (e.g., echo, print)

Related Questions:

• Headers already sent by PHP
• All PHP "Headers already sent" Questions on Stackoverflow
• Byte Order Mark
• What PHP Functions Create Output?

The above is the detailed content of Why Am I Getting the PHP Warning: 'Cannot modify header information - headers already sent'?. For more information, please follow other related articles on the PHP Chinese website!