Sometimes you might need to exclude a single file from your project, so that it will be ignored by inspections, code completion, etc. This is done using the Mark as plain text action.
When a file is marked as plain text, PhpStorm does not use it anymore for code completion and navigation. Such file is shown as plain text in the editor, and is denoted with a special icon plain_text_icon in the Project tool window.
http://www.jetbrains.com/phpstorm/webhelp/excluding-files-from-project.html