Add test with quoting multibyte GBK vs utf8mb4 in PDO

ext/pdo_mysql/tests/pdo_mysql_quote_gbk_double_escape.phpt

@@ -0,0 +1,44 @@
+--TEST--
+PDO_MYSQL: Test quoting of multibyte sequence with GBK vs utf8mb4
+--EXTENSIONS--
+pdo_mysql
+--SKIPIF--
+<?php
+require_once __DIR__ . '/inc/mysql_pdo_test.inc';
+MySQLPDOTest::skip();
+?>
+--FILE--
+<?php
+    require_once __DIR__ . '/inc/mysql_pdo_test.inc';
+
+    $link = MySQLPDOTest::factory('PDO', ['charset' => 'GBK']);
+    $quoted = $link->quote("\xbf\x27");
+    $quoted_without_outer_quotes = substr($quoted, 1, -1);
+
+    /* This should result in 5C BF 5C 27 for GBK instead of BF 5C 27 like with UTF8MB4.
+     * To explain why the extra escaping takes place, let's assume we don't do that and see what happens.
+     *
+     * 1. First iteration, i.e. *from == 0xBF. This isn't a valid GBK multibyte sequence start,
+     *    so the mb validity check fails.
+     *    Without the character length check, we'd check if we need to escape the current character 0xBF.
+     *    The character 0xBF isn't handled in the switch case so we don't escape it and append 0xBF to the output buffer.
+     * 2. Second iteration, i.e. *from == 0x27. This isn't a valid start either, so we go to the escape logic.
+     *     Note that 0x27 is the character ', so we have to escape! We write two bytes to the output:
+     *     \ (this is 0x5C) and ' (this is 0x27).
+     * 3. The function finished, let's look at the output: 0xBF 0x5C 0x27.
+     *    Now we actually made a problem: 0xBF 0x5C is a valid GBK multibyte sequence!
+     *    So we transformed an invalid multibyte sequences into a valid one, potentially corrupting data.
+     *    The solution is to check whether it could have been part of a multibyte sequence, but the checks are less strict. */
+    var_dump(bin2hex($quoted_without_outer_quotes));
+
+    unset($link);
+
+    // Compare with utf8mb4
+    $link = MySQLPDOTest::factory('PDO', ['charset' => 'utf8mb4']);
+    $quoted = $link->quote("\xbf\x27");
+    $quoted_without_outer_quotes = substr($quoted, 1, -1);
+    var_dump(bin2hex($quoted_without_outer_quotes));
+?>
+--EXPECT--
+string(8) "5cbf5c27"
+string(6) "bf5c27"