Support for language highlights embedded in PHP code

[zonuexe]

Query languages like SQL and, rarely, HTML tags can be written in string literals and heredoc, so it's useful to have syntax highlighting

<?php

$sql = 'SELECT * FROM tbl';

if (true) {
    $sql = <<<'SQL'
    SELECT * FROM tbl
    SQL;
}

refs emacs-php/php-mode#695