New sniff: change heredoc to nowdoc if no variables are used

[jrfnl]

Sniff basics	-
Fixable for PHP:	5.3+
Sniff type:	Modernize
Fixer type:	Risky

Short description

PHP 5.3 introduced the nowdoc syntax. When no variable extrapolation or escape sequences are needed, a nowdoc should be used instead of a heredoc.

Related PHPCompatibility sniff(s):

• Newkeywords

PHP manual references:

• Nowdocs are to single-quoted strings what heredocs are to double-quoted strings. A nowdoc is specified similarly to a heredoc, but no parsing is done inside a nowdoc.

  http://php.net/manual/en/language.types.string.php#language.types.string.syntax.nowdoc

• http://php.net/manual/en/language.types.string.php#language.types.string.syntax.double (escape codes)

Example code:

Detect the following code pattern(s):

$str = <<<EOD
Example of string
spanning multiple lines
using heredoc syntax.
EOD;

// This heredoc should not be touched by the sniff!
echo <<<EOT
My name is "$name". I am printing some $foo->foo.
Now, I am printing some {$foo->bar[1]}.
This should print a capital 'A': \x41
EOT;

And fix these to:

$str = <<<'EOD'
Example of string
spanning multiple lines
using heredoc syntax.
EOD;

// Second example should be untouched.

Notes for implementation of the sniff:

• Beware of escaped variables in heredocs \$variables. These will not be extrapolated.
• Beware of escape sequences which necessitate the use of heredoc over nowdoc.

Prior art

• CS-fixer: heredoc_to_nowdoc