Sourcecode requires special handling, layout and storage, to prevent inadvertent merging of source files with the code running the wiki, the browser, and the database.

To display formatting existing in source files, the file is enclosed with pre/code tags.   The pre tags forces the browser to display the original whitespace in the code.   The code tag is formatted with CSS to display a distinct background and border.

Code is also easier to read when highlighted in some way.
There are scripts to provide syntax highlighting, and a built in function (highlight) for PHP.