[php] Add extern for `print_r()` (#9834)

std/php/Global.hx

@@ -295,6 +295,11 @@ extern class Global {
 	**/
 	static function echo(args:Rest<String>):Void;
 
+	/**
+		@see http://php.net/manual/en/function.print-r.php
+	**/
+	static function print_r(expression:Any, ?returnOutput:Bool):EitherType<Bool, String>;
+
 	/**
 		@see http://php.net/manual/en/function.sprintf.php
 	**/