File Extensions (C++ and PHP Cross Reference Guide)

Important standard C++ file extensions:

• .CPP = C++ Source file. Your startup source file will have a main() routine.
• .C = C source file (sometimes used for C++ source files too).
• .H = Header include file.

Some important Visual C++ file extensions:

• .SLN = Visual Studio.Net solution
• .VCPRJ = The Visual C++ project file. See Project Files and Makefiles for more information.

Some important C++Builder file extensions:

• .BDSPROJ and .CBPROJ = Project file.

 Ref: http://msdn.microsoft.com/en-us/library/3awe4781(VS.80).aspx

.php is the default extension for PHP although some developers will change the default extension in an effort to add an additional security level. If your code is tied to a particular version of PHP then some developers at the major PHP version number to the extension as in .php3, .php4, .php5, etc. 

.phtml is also sometimes used especially for files that contain both HTML and Perl code.