A compiled language C or C++ - the history of programs like evolution, which suffered extensive delays and has been the driving force of miguel (from what I've read/heard) moving toward C# is evidence that the concept of developing all of a large scale application in only a high level language is fraught with problems - generally thought to be problems with memory management,  (Im not actually sure that C# will prove a much better alternative).

Java and C# , bytecode compiled languages, object orientated languages with strict variable definitions, again, these offer a few benifits over the high level languages, where the reduction of memory management consideration in coding improves productivity. It's just since C# is not ready on linux yet, and Java is a pain to install on debian (thanks to suns licencing), And from a long experience of getting Java going on different platforms it was obvious that trusting MS to keep java running correctly on windows was a recipe for disaster!.

Scripted, loose typed (non decleared variabled), runtime bytecode compiled languages like Python, Pike, Perl, and Ruby - These are extremely good as prototyping languages where the slower bits can be speeded up by writing extenstions in C etc. Almost all of these languages have similar advantages, Perl and PHP have strong variable identifiers, ruby and pike have strong object orientation. - noteable in .Net's hidden adgenda appears to make all these language imposilbe to integrate into the .NET CLR by design (as it has  design critera that alienates non declared variable languages)!

Others like delphi - which is not open souce, or particualy stable on linux....
lisp and it's friends - which after having spent 3 years writing scripts/applications in, I never would want to touch again.

Having developed in Pike,Python and Perl
(they do have their good points, but these are the ones that are -for me - significant downsides)
Perl - write once, never touch again - it is near impossible to write clear readable perl = even a 20 line shell script is a pain to work with later.
Pike - nice language, but it has a very small following and limited documenation (not critical mass)
Python - unclear object instantation referencing - difficult to read if you are using a static or dynamic class reference. (I believe that has improved, but it was increadibly fustrating to deal with) - probably the only other serious competition to PHP....
Ruby - again looks like another very nice language, similar to pike, but the big issue is that critical mass is important as it affects the quality and quantity of both documentation, extensions and sample code that are made available.


So in chosing a language for phpmole, these where the factors that where important.
1. Speed of development - no double compile/execute process
2. Documentation - online (well at least for the main language)!
3. Clarity of reading pure code - following most of PEAR rules tends to make PHP that reads like a book.
4. Object orientation - just the basics!
5. Source code! - the language source was written and organized in a clear enough way that it could be fixed if necessary. = no unexpected problems.
6. Speed - since there only a small load up 'compile' penalty on the language start up, after that it behaves like any other bytecode language.
7. Similarity to C - the extension language - making it easy in theory to build accelerate a section by rewriting it in C.
8. Sample code and libraries already written that can be extended. - Although most people think of PHP as a few prints in a web page, it has a much better written set of libraries in PEAR.php.net and a big black hole of libraries from phpclasses.upperdesign.com.
9. large existing userbase - which contributes to 2 & 8 and may even contribute to the mole :)
10. cross platform - other than Ada (which I've never tried), the only other option would be java which has the same downsides as delphi in stability on the desktop.

Things that where not available that are not important in 98% of development - and make life confusing anyway..
1. Weird and wonderfull Object orientation and functional features - polymorphism and all those other much banded catchphrases but totally useless for rapid applicatoin development and tend to cause more problems than they solve.

The only downside that I have found so far in PHP GTK is the lack of thread support , for the most part this can be worked around (using gtk::events_pending() calls, or forking(pcntl), sockets and a bit of intelligence...), - adding threading obviously has significant downsides is that it is a large black hole of introducing bugs from poor variable locking as well....  = give yourself enough rope ......

Obviously with PHP5 , the multiple interitance (using aggregation), exceptions - catch throw, and yet more extensions available, It will make developing applications like phpmole will become even easier.

As far as design, from my impressions - PHP is designed to be similar to C/C++, but with variable identifiers and a simplified 'user friendly' object orientation.

anyway a discussion like this could go on for ever....