Programming archaeology: PHP's dollar sigil

I started out with Java as my professional language and before I discovered Ruby, PHP was my main programming language of choice for new projects. This was probably six to seven years ago or more. I had some pretty good times with CodeIgniter [0] and various libraries like HHMVC [1].

In any case, I digress - even back then the most striking feature of its syntax was the dollar sign prepended to variables which made me think the aesthetics of the language were sub-par or at least annoying. Clearly it did not stop me from writing lots of apps in PHP since it was more of a small annoyance, if you will, something debatable mostly for scholastic purposes.

Recently the question popped into my mind: "What is the history of the dollar sign, and why is it there?" - so like every good question I started to Google my way into some answers:

From Perl to PHP

First influence - Perl:

Perl might have inherited this from shell tools like awk et al. hence:

A Quora answer provided a useful anecdote that further connected the ideas above:

Sigils in Perl

From this we find out that $, @ and % are actually called sigils [2] in computer programming. Next stop was to see what these sigils indicated in Perl:

Scalar values are always named with '$', even when referring to a scalar that is part of an array or a hash. The '$' symbol works semantically like the English word "the" in that it indicates a single value is expected.

  $days               # the simple scalar value "days"
  $days[28]           # the 29th element of array @days
  $days{'Feb'}        # the 'Feb' value from hash %days
  $#days              # the last index of array @days

Entire arrays (and slices of arrays and hashes) are denoted by '@', which works much as the word "these" or "those" does in English, in that it indicates multiple values are expected.

  @days               # ($days[0], $days[1],... $days[n])
  @days[3,4,5]        # same as ($days[3],$days[4],$days[5])
  @days{'a','c'}      # same as ($days{'a'},$days{'c'})

Entire hashes are denoted by '%':

  %days               # (key1, val1, key2, val2 ...)

via http://search.cpan.org/~shay/perl-5.24.1/pod/perldata.pod#Variable_names

A Ruby comparison

Very interesting and they remind me of Ruby's shorthand or general delimited input:

%w(foo bar) # => ["foo", "bar"]
# %r() is another way to write a regular expression.
# %q() is another way to write a single-quoted string (and can be multi-line, which is useful)
# %Q() gives a double-quoted string
# %x() is a shell command
# %i() gives an array of symbols (Ruby >= 2.0.0)
# %s() turns foo into a symbol (:foo)

Conclusion

Conclusion? The dollar sign for variables in PHP is actually a vestigial sigil since compared to Perl there aren't any for arrays and hashes.

[0] - https://codeigniter.com [1] - https://github.com/jenssegers/codeigniter-hmvc-modules [2] - https://en.wikipedia.org/wiki/Sigil_(computer_programming)

This historical chain is reconstructed from secondary and Q&A sources; it is a plausible trail, not primary-source proof of PHP’s design lineage.