php: Add syntax highlighting inside heredoc strings

[sylbru]

PHP heredoc strings make it easy to define string literals over multiple lines:

    $someString = <<<EOT
        multiline
        text
        EOT;

That EOT identifier can be anything else, and it is actually being used in Sublime Text and VS Code to inject syntax highlighting for another language in said string, depending on the identifier. For instance, if the identifier is SQL, SQL syntax highlighting will be applied to the contents of the string. Likewise if the identifier is CSS or JS.

    $someString = <<<SQL
        SELECT *
        FROM my_table
        SQL;

This PR changes the PHP extension so that it supports that feature too.

Release Notes:

• php: Added syntax highlighting inside heredoc strings

[cla-bot]

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[sylbru]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[SomeoneToIgnore]

Thank you.