Description:
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In Perl, we could write the following program test.pl (lines are enumerated for better look):

01  #!/usr/bin/perl -w
02  print "hello!\n";
03  print "something!\n";
04
05  #line 100 "aaa.pl"
06  print $undefined_var;

This code generates a warning message "Using of undefined variable", but (attention!) - the context of this error is NOT "test.pl line 5", but - "aaa.pl line 100".

So, the "pseudo-comment" operator

#line xxx "yyy"

in Perl substitutes the current parsing context to a specified line number and filename. (You may ommit filename, then - only the line number will be substituted). And, if a warning or a fatal error occurs, the line number and filename in the error message will be changed. There is the same feature in C language: it is used by C preprocessor to execute #include directives an keep line numbers clear to understand.

This feature is VERY VERY useful when we write some code translator (e.g. from a templating language to pure PHP). This kind of translation is done e.g. in Smarty: it translates Smarty templates into pure PHP code, but line numbers are mixed up in translated files, so we cannot revert back and detect which line of the template had produced an error.

Please say, do you plan to add this feature to a new PHP versions? I'd like to ask you very much about it...