What is the file?
You either have an HTML document generated using PHP, in which case rename it so it has a .html file extension and open it in a web browser. Or you have some PHP program code, in which case you need to place it on a webserver with PHP installed on it. A third alternative is similar to the first, except that rather then generating a webpage, it provided a different type of file (such as a PDF, executable or graphic) - rename it so the file extension matches the type you were expecting. From what you've said it doesn't seem that Acrobat Reader recognises it so I suspect it's not a .pdf file. Change the suffix to .html and see if it will open in your browser (IE< Firefox etc.)