Plugin files can either be located directly in the wp-content/plugins directory or in a subdirectory under this location. When you access the installed plugins list in the administration interface, WordPress scans all potential plugin locations, looking for PHP files that contain comments following the format specified in this recipe. There can actually be one or more PHP files containing plugin header data in any of these directories, and each of them will show up as an entry in the plugin list.

Taking a closer look at the code that we entered in the file, the first line of the plugin file is a tag that identifies the beginning of the PHP code that will be analyzed and executed by the PHP interpreter. Optionally, we could include a closing PHP tag (?>) at the end of the file. However, most PHP developers omit the closing tag, since having any spaces after that tag will cause warnings to be displayed by the interpreter.

The second and last lines indicate that the enclosed text should be considered as text comments. Finally, each line within the comment contains a specific label, indicating the type of information that follows it. When this information is found, WordPress retrieves data about the plugin and adds it to the list.

When a plugin is activated, WordPress validates the file's content to be sure that it is valid PHP code. It will then execute this content every time any page is rendered on the site, whether that page is front-facing or a backend administration section. For this reason, it is preferable to activate plugins only when they are in use, to avoid site slowdowns.

Of course, at this point, our new plugin does not add or modify any functionality in our WordPress installation, since it does not contain real code, but this is still an important first step.