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Not used anywhere in Piwik core because we use Zend to prepare statements. Moreover, manual for PDO::quote says:
[...] you are strongly recommended to use PDO::prepare() to prepare SQL statements with bound parameters instead of using PDO::quote() to interpolate user input into a SQL statement. Prepared statements with bound parameters are not only more portable, more convenient, immune to SQL injection, but are often much faster to execute than interpolated queries, as both the server and client side can cache a compiled form of the query.
(In ) fixes #882 - Piwik_Query() can now be used by tracker plugins;
fixes #1292 - remove (deprecated/unused) Piwik_Quote() since we use Zend_Db for prepared statements; add Piwik_FetchRow() for completeness
(In [refs #1292/2054) - typo in phpdocs