PHP already has case sensitive variable names. $Foo and $foo are always different variables.
Function names, class names, keywords (class, function, extends, if, while, etc) are always case insensitive.
However, constants are sometimes case sensitive, depending on their declaration.
I do a lot of PHP development, but these days it's only sane by the fact that I've been doing it so long I understand many of it's weirdness. Also, using frameworks (Symfony 1 & 2) and finally using a template engine (Twig) helps enormously. Helps in the same way jQuery has saved me from writing vanilla JS and trying to deal with browser quirks.