Never close PHP class files with the “?>” tag

When developing PHP, a front-end PHP file will include other files: classes, utilities, etc.

When writing those class files, one also needs to use the <?php tag at the start of the file, otherwise PHP will simply take the text and output it unchanged to the browser. (PHP's assumption is that it sits in a web page, with probably more markup than code, so by default characters in the source code get copied one-to-one to the browser and the <?php...?> tags are necessary to introduce PHP to the "exceptional circumstance" that one might actually want to program some PHP.)

If one must open the class source file with <?php then it would seem to make aesthetic sense to close it with ?>. However, there are no negative side-effects if one does not close the tag, plus one very negative side-effect if one does close it.

We performed a minor release a while ago, after which the display of generated PDF files no longer worked. Yet the minor release had nothing to do with the section of code that produced PDFs. What sort of weird action-at-a-distance could possibly be happening here?

The reason was that one class file in the minor release had a blank line after the ?> tag. This was impossible to spot in the text editor. The blank line was printed to the browser, which was also invisible in nearly all of the site, as HTML ignores blank lines. PDFs probably do as well (I haven't checked) but the problem wasn't with the content. As HTTP response content is streamed to the browser (as opposed to being collected first and then sent to the browser at the end of the request), HTTP headers can only be set before the first byte of output has been produced by the software. As the blank line in the class source file consituted content, and the source file was (necessarily) parsed before the code could be executed, the HTTP header "Content-Type: text/pdf" couldn't be sent, and various errors about headers not being able to be sent, combined with the binary source of the PDF, arrived at the user's screen.

So given there are no disadvantages, and one particulary weird source of bugs can be removed, I would say one should never end PHP files with ?>.