MDL-49059 medialib: Support start time params in Youtube URLs

Medialib (and thus the Multimedia Plugins filter) was ignoring start time
parameters on YouTube video URLs, meaning that the video would always
start at the start, even when the appropriate parameter was present.

lib/medialib.php

@@ -536,15 +536,62 @@ class core_media_player_youtube extends core_media_player_external {
 
         self::pick_video_size($width, $height);
 
+        $params = '';
+        $start = self::get_start_time($url);
+        if ($start > 0) {
+            $params .= "start=$start&";
+        }
+
         return <<<OET
 <span class="mediaplugin mediaplugin_youtube">
 <iframe title="$info" width="$width" height="$height"
-  src="https://www.youtube.com/embed/$videoid?rel=0&wmode=transparent" frameborder="0" allowfullscreen="1"></iframe>
+  src="https://www.youtube.com/embed/$videoid?{$params}rel=0&wmode=transparent" frameborder="0" allowfullscreen="1"></iframe>
 </span>
 OET;
 
     }
 
+    /**
+     * Check for start time parameter.  Note that it's in hours/mins/secs in the URL,
+     * but the embedded player takes only a number of seconds as the "start" parameter.
+     * @param moodle_url $url URL of video to be embedded.
+     * @return int Number of seconds video should start at.
+     */
+    protected static function get_start_time($url) {
+        $matches = array();
+        $seconds = 0;
+
+        $rawtime = $url->param('t');
+        if (empty($rawtime)) {
+            $rawtime = $url->param('start');
+        }
+
+        if (is_numeric($rawtime)) {
+            // Start time already specified as a number of seconds; ensure it's an integer.
+            $seconds = $rawtime;
+        } else if (preg_match('/(\d+?h)?(\d+?m)?(\d+?s)?/i', $rawtime, $matches)) {
+            // Convert into a raw number of seconds, as that's all embedded players accept.
+            for ($i = 1; $i < count($matches); $i++) {
+                if (empty($matches[$i])) {
+                    continue;
+                }
+                $part = str_split($matches[$i], strlen($matches[$i]) - 1);
+                switch ($part[1]) {
+                    case 'h':
+                        $seconds += 3600 * $part[0];
+                        break;
+                    case 'm':
+                        $seconds += 60 * $part[0];
+                        break;
+                    default:
+                        $seconds += $part[0];
+                }
+            }
+        }
+
+        return intval($seconds);
+    }
+
     protected function get_regex() {
         // Regex for standard youtube link
          $link = '(youtube(-nocookie)?\.com/(?:watch\?v=|v/))';