MDL-28676 add hack mysqli php ext requirement for upgrades to 2.x

The mysql extension is not supported in Moodle 2.x, this hack should inform admins that they must add mysqli support before doing 2.x upgrades.

lib/environmentlib.php

@@ -482,6 +482,17 @@ function environment_check_php_extensions($version) {
         $result->setErrorCode(NO_PHP_EXTENSIONS_SECTION_FOUND);
         return array($result);
     }
+
+/// Hack alert: inject extra mysqli dependency, mysql PHP extension is not supported in Moodle 2.x, warn only ppl using MySQL.
+    global $CFG;
+    set_dbfamily();
+    if ($CFG->dbfamily === 'mysql') {
+        $mver = normalize_version($version);
+        if (version_compare($mver, '2.0', 'ge')) {
+            $data['#']['PHP_EXTENSIONS']['0']['#']['PHP_EXTENSION'][] = array('#' => '', '@' => array('name' => 'mysqli', 'level' => 'required'));
+        }
+    }
+
 /// Iterate over extensions checking them and creating the needed environment_results
     foreach($data['#']['PHP_EXTENSIONS']['0']['#']['PHP_EXTENSION'] as $extension) {
         $result = new environment_results('php_extension');