As we know, a web page is made up of two parts – the page header and the body. When a web developer incorrectly creates or modifies a page header, he may see one of the common PHP errors. The error states “Warning: Cannot modify header information – headers already sent by …” with details of the file and line of code with the error. If the developer is unaware of the cause of this error, he may spend hours to get the issue resolved. Understanding why the error occurs will help you find the solution.