Fix and improve exporting in PHP format

[mlocati]

In PHP we have two kind of strings (well, excluding heredoc/nowdoc strings):

• single-quoted strings (eg 'Hi there')
• double-quoted strings (eg "Hi there")

They behave very differently:

• single-quoted strings can't contain special character sequences (except \' and \\)
• double-quoted strings can contain sequences (for example \n), as well as variables (example)

This, for example, implies that we have to "escape" the $ in double quoted strings, otherwise we break the strings (and possibly the execution - example).

Furthermore, since PHP 5.4 (which is 12 years old), PHP supports the compact [...] notation for arrays (instead of the more verbose array(...)).

Finally, accordingly to the PSR-2 (PHP Standard Recommendation # 2), NULL values should be written in lower case (see here).

So, we can't really use the same function to generate JSON and PHP strings.

In this PR I renamed the old FormatPhp function to FormatJson, and implemented a new FormatPhp function that:

• exports strings in single-quote format, if possible
• escapes more characters if the double-quote format is used

PS: I don't have Delphi, so I didn't test anything (and it has been 3 decades since the last time I wrote some Pascal code 😉) - so please check that everything works as expected.