[support] php based themes

[Mohammed al-shar']

dear all,
hello.
I noticed that there are some themes made to work with the xtemplate engine in versions prior to 4.7, another type that works with the phptemplate engine, the default for 4.7 and smart theme engine. 
yet, some themes like Goofy, Slash, etc. are php based themes, the file that sets the layout in them is a file called file.theme 
before I go on with what I want to achieve, can anybody explain the last type to me, and tell me where it relates?
thanks.
mohammed.
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