docs: add link to php.net

Author: kenjis

user_guide_src/source/libraries/time.rst

@@ -22,8 +22,12 @@ Instantiating
 *************
 
 There are several ways that a new Time instance can be created. The first is simply to create a new instance
-like any other class. When you do it this way, you can pass in a string representing the desired time. This can
-be any string that PHP's strtotime function can parse:
+like any other class.
+
+When you do it this way, you can pass in a string representing the desired time. This can
+be any string that PHP's `strtotime()`_ function can parse:
+
+.. _strtotime(): https://www.php.net/manual/en/function.strtotime.php
 
 .. literalinclude:: time/001.php