Canvas is not a file format, so there's no such thing as saving a drawing in a "Canvas file." Of course, a web designer could easily store Canvas commands in a file and load them via PHP.
Canvas is a project of the Web Hypertext Application Technology Working Group (WHATWG), a "loose unofficial collaboration of Web browser manufacturers and interested parties who wish to develop new technologies designed to allow authors to write and deploy Applications over the World Wide Web."
For more information, see the Mozilla Developer Center's excellent article Drawing Graphics with Canvas.
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