[fetbaffe]

$ was taken from Perl

https://stackoverflow.com/a/3073818

but because early PHP was more simplistic than Perl it only has $.

Powershell also uses $ for variables.

Regardless of etymology of the $ sign and the usefulness of it, personally I like it because it makes it easier for me when I'm reading code, especially if you are scanning fast, to differentiate variables from symbols. Yes, you can get that with your editor too, but for me it is easier to associate the $ sign with a variable rather than a specific color, sometimes you don't have coloring available like when in command line going thru diffs, cat, nano etc.

[jhgb]

It seemed kind of obvious to me even at that time that it was taken from Perl. I just didn't get why. PHP could have lifted IMPLEMENTATION DIVISION from COBOL, but likewise, there would have been no point either (fortunately it didn't do that!).

[fetbaffe]

Early web was written in Perl, I guess it was the classic approach of lets borrow what is already successful and in end PHP won over Perl so it worked too. Bill Gates would have been proud.