Add a code example for comments
Clarify the comments section

- Why would we want to use comments in the first place?
  - To communicate with human beings
  - To temporarily disable programming statements
- Both syntaxes of single line comments do the same thing
Link PHP.net, and briefly explain what it is
Variables are created, referenced with same syntax
Add single/double quoted string example
Replace "scalar" with "dynamically typed"

Also update example to include "output" in comments
Replace awkward language around '!' as logical not
Fix logical and/or example
Include PHP label rules blurb with variables

Link to this blurb from the user defined function section.
Fix awkward sentence.
Add warning to avoid using `global`
Explain the <= operator
Add pronunciation of MySQL

Also fix a few typos.
Remove "self-explanatory" note on INSERT

Of course, this may not be self-explanatory to students.
Replace paralanguage phrase with something simpler

Also add a note about using = in SQL for comparison, compared with == or
=== in PHP.