For information on function pointers in C: function-pointer.org
A nice book on OCaml is free online: O'Reilly's Developing Applications with Objective Caml
A really weird introduction to Ruby is Why's Poignant Guide to Ruby
The first edition of Programming Ruby is available online as well.
A decent introduction to regular expressions is on regular-expressions.info.
For PHP, nothing's better than the manual.
If you want to learn about Scheme (or computer programming in general, really), the ever popular "Structure and Interpretation of Computer Programs" from MIT is available online (http://mitpress.mit.edu/sicp/full-text/book/book.html) and has accompanying video lectures!
Also of use to lots of other things is MIT's OpenCourseWare.




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