I just found PHPAnywhere: http://phpanywhere.net/
It looks pretty impressive, I will probably check it myself. You can learn ECCO: http://ecco.sourceforge.net/ - Of course, the advantage here is that it is open source and downloadable, that if I don't read too much in my question, this is what you want to. Apparently, this is in Alpha status, but the screenshots suggest that it is at least moderately functional.
Mozilla Labs' Bespin, an online HTML editor, also appeared on my search, but so far I havenโt. um, get it.
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