once you have gotten into game developement, you will find very quickly, that performance matters ... dynamic languages are slow ... and PHP, as you mentioned, is by far one of the worst ... plus the language is inconsistent and the API is horrible ... which is why there is no proper IDE for that ...
ok, personally, i think the AVM2 (actionscript virtual machine 2, i.e. flash player 9 and higher), is a good platform for web based game developement ... as you are a windows user, i would advise you to use Flash Develop, as Bryan did ... it's simply the BEST ActionScript IDE around (ok, some guys might argue that FDT is better, but ... no ... plus FD is free, and FDT is at fewhundred dollars) ... it's so good, that mac and linux users virtualize windows to run it ... an ass-kicking autocompletion, code generation, snippets etc. ... and it's fast, unlike other IDEs providing these features (it feels much better than eclipse and net beans)...
FlashDevelop is a plain ActionScript editor ... unlike in the flash IDE, you can't use it for drawing/morphing/tweening ... but you can use FD to write your code and the flash IDE to do the graphics and then compile either with FD or the flash IDE (the latter is easier, since there is a project type that offers a very good work flow) ...
you could alternatively use haXe, which is a high level open source language, that allows compiling/translating to AVM1 bytecode, AVM2 bytecode, PHP source, JavaScript source, neko vm bytecode, C++ source (.NET and JVM are being worked on) ... it also has very cool languages features ... but it requires you to code even stricter, than ActionScript ... on the other hand, the compiler outputs bytecode, that is upto 30% faster ... and haXe can use Flash Player 10 alchemy opcode, that allow very fast and direct memory manipulation ... and interesting fact to mention is that haXe was developed by Nicolas Canasse from motion twin, who are the leading web game developer/publisher in France ...
choosing flash as a plattform seems to make sense to me, because flash is
- fast
- small
- has been used A LOT for web games, so there are PLENTY of resources around, you can use, like physics engines (APE for actionscript, physaxe for haXe), etc. ...
the main flash con to me is, it has performance problems on mac and linux ... but anything that is not developed for mac or for linux respectively, faces these problems ...
about other platforms mentioned:
- JavaScript: you will REALLY hate this ... every browser implements everything differently ... you will have an awful time ... canvas is not available across all browsers ...
- Java: it's suprisingly slow in relation to its verbosity, bloated, and it's the wrong language for games ... use JavaFX instead ... but still ... personally i don't like software running on the JVM, simply because of the VMs insane booting time ...
- Silverlight: in principle, a VERY promising technology, but penetration is still relatively poor and it has practically never been used for web games, so you can't really rely on years of experience of many developers ... i think there are reasons why silverlight did not (yet?) turn out to be the flash killer, although there is a huge .NET developer community ...
- gamemaker: it's an extra plugin ... don't know, if that's cool with you ... but i like to use things that are widely spread ... if you wanna follow that path, you might wanna check out unity as well ... note that both are for creating 3d games, which is not what you wanted ... so they don't really offer an advantage over flash ...
in any way, you should use a technology and language, that A is developing, and B you are comfortable with, or that you find really interesting enough, to get comfortable with it ...
well, good luck then ... ;)