Minor - fixing PHP warning

main/document/download.php

@@ -31,10 +31,10 @@ if (strpos($doc_url, '../') OR strpos($doc_url, '/..')) {
 }
 
 // Dealing with image included into survey: when users receive a link towards a
-// survey while not being authenticated on the plateform.
+// survey while not being authenticated on the platform.
 // The administrator should probably be able to disable this code through admin
 // inteface.
-$refer_script = strrchr($_SERVER["HTTP_REFERER"], '/');
+$refer_script = isset($_SERVER["HTTP_REFERER"]) ? strrchr($_SERVER["HTTP_REFERER"], '/') : null;
 
 $sys_course_path = api_get_path(SYS_COURSE_PATH).$_course['path'].'/document';