This will be a quick post/note-to-self since I’m pretty occupied. Actually, the title of this post should have been “How to bloody fix the deceptively easy but hard to find confounding error in PHP headers: “warning Cannot modify header information – headers already sent”. But that’s too bloody long (though it would be interesting to find out in the future how WordPress concatenates long URLs…). The reason why I call it deceptive will be clarified at the 3rd entry below 🙂
The 2 primary parts of an HTTP request response are the headers and the body, which should be sent separately. Now in PHP sometimes some programmers, not just novice ones but long time ones (ehem…like me…) forget that we’re modifying them programmatically, which can sometimes cause errors. The header must always be sent first before the body, wherein both are coming from the web server. This is highlighted in this Wikipedia HTTP request example. For example, the php function header() can modify some of the (obviously) header parameters, most/all of which are listed in this Wikipedia list of HTTP headers. The above PHP error occurs because the body (or part of it) has already been sent by the server to the client, afterwhich a change of header values follows, either from the client or server.
Now, to finally fix the deceptively easy to fix but hard to find source of the error
Warning: Cannot modify header information – headers already sent by (<some of your PHP source files should be listed here>)
You can check the following:
1) If you’re using the header() function or some PHP function that modifies the header or controls the flow of action of your pages (e.g. from one page to another), you should inspect those. Usually it’s better to use conditional statements (e.g. IF or ELSE) to isolate the execution of one part of your code from another. It is quite likely that the error comes before or at the line of this function.
2) Make sure you don’t output/print/echo anything to the client (body) before sending/changing the headers. Again, conditional statements are useful here.
3) Finally, and the easiest to overlook, is to remove any white space outside the PHP start and end tags (<?php ?>). This is quite often the easiest thing to miss (for me at the least). The reason for this deceptive white space causing an error in PHP is that the white space is still interpreted as an echo statement printing a blank line, which interrupts the format of the HTTP header (see Wikipedia header format example above).
Of course the disclaimer here is that you are to most likely encounter this error if you’re more or less building your PHP application from the ground up, or without using web frameworks. It’s more unlikely and unusual to receive this error while you’re using an MVC based PHP framework.