Search syntax broken when using Sphinx Fulltext backend

Fix for https://tracker.phpbb.com/browse/PHPBB3-15367 used EscapeString function to escape special characters in search queries passed through to the Sphinx engine.

This means that the following special characters used in phpBB search queries are fed through to Sphinx as literal characters, breaking their functionality. Currently, those search characters are simply ignored by the search, as are the words NOT and AND.

As a result the following characters do not work as advertised on phpBB search when using Sphinx Fulltext as the search backend:

+

-

"

*

Resolving this issue would require creating a custom function to escape only those characters which require escaping to avoid search errors, eg /, $.

In fixing this issue, we will need to avoid a situation where users searching for hyphenated words will have the following word excluded from search results. Currently hyphens are simply ignored.

Resolution of this issue should result in exact phrase searching being possible on phpBB, as described here: https://tracker.phpbb.com/browse/PHPBB3-16233

Instruction text on search.php also needs updated to properly reflect actual search functionality, including possibility of exact phrases when using Sphinx or mySQL.