This feature is badly lacking to support many good, modern patterns, which could broaden the scope and use of PHP in general. View-engines as mentioned, but not least aspect-oriented programming, code generation and meta-programming in general, and cross-language-compilation from other languages.

Microsoft's C# compiler has an implementation of this feature that could be referenced:

http://msdn.microsoft.com/en-us/library/aa691102%28v=VS.71%29.aspx

I would propose extending the existing syntax for execution directives, as implemented by the declare keyword:

http://www.php.net/manual/en/control-structures.declare.php

It might looks like this:

  // start of file
  declare(source='test.c#20');
  something();
  
  declare(source='test.c#30') {
    other();
    more();
  }
  // end of file

Consistent with the way declare otherwise works - the first declare-statement in this example changes the error-reporting for all of the following statements in that file, while the second declare-statement changes the error reporting within the brackets.

I think this idea should be taken into serious consideration, as it has the potential to open up the world of PHP to entirely new forms of development.

Thank you for your interest in PHP and for submitting a feature request. Please
be aware that due to the magnitude of change this request requires, it would be
necessary to discuss it on PHP Internals list (internals@lists.php.net) as an
RFC. Please read the guide about creating RFCs here:
<https://wiki.php.net/rfc/howto>. If you haven't had experience with writing
RFCs before, it is advised to seek guidance on the Internals list
(<http://php.net/mailing-lists.php>) and/or solicit help from one of the
experienced developers. 

Please do not consider this comment as a negative view on the merits of your
proposal – every proposal which requires changes of certain magnitude, even the
very successful and widely supported ones, must be done through the RFC process.
This helps make the process predictable, transparent and accessible to all
developers.