I suspect the dollar signs may be part of the reason of PHP's success. While it's clear for experienced programmers that "x" and "y" are variables and "foo" is a function in "x = foo(y)", for new programmers "$x = foo($y)" is probably easier to read and mentally parse.

That is my personal experience anyway, as someone whose first truly serious language was PHP 20 years ago. Maybe it was just because I was used to it (BASIC also used $), but it took me a bit to get used to the "bare" variables in Python and C back in the day.