coding style change: use single quotes

In all PHP code, use single quotes instead of usual quotes where possible,
i.e. when no variable is embedded in a string. Because these single-quoted
strings do not need to be searched for variables by PHP, they're supposed
to be faster.

Also, avoid as many escaped quotes (\") as possible by alternating between
single and double quotes.
rename / recase some array fields
array fields embedded in strings

Use a consistent style for embedding array fields in strings:
"string with $array[field]";
Only use {$...} if it is needed.
correct 2 license links
Merge branch 'coding-style-fixes'