These all are the extensions, that indicate about the file. .asp- It is an extension or filename for the pages that are designed in ASP. ASP stands for Active Server Pages and it is server-side scripting languge.
.aspx- It is the extension or the filename for the page that is designed in ASP.NET. ASPX stands for Active Server Pages Extended and it is the server-side Web Application Framework. It is used for the Dynamic Web pages.
.php- It is the extension or filename for the page that is designed in PHP. PHP is server-side scripting language and widely used for web development and for general purpose. Recursive Acronym for PHP is Hypertext Preprocessor. The main advantage of the PHP is, it is open source.
.html- .html extension is used for the webpages that are designed in HTML. HTML stands for Hypertext Markup Lamguage. HTML is the standard markup language for designing the web pages.
.aspx is the extension for Active Server Page Extended(ASPE) Files.
.asp is the extension for the classic pages designed in Active Server Pages(ASP).
.php is the extension for the pages designed in Hypertext Preprocessor(PHP).
.html is the extension for the pages designed in Hypertext Markup Language(HTML).