The merge parameter respects array keys, so if you merge two numerically indexed arrays, they may overwrite each other or result in non-sequential keys. This is unlike the array_merge() function of PHP which wipes out numerical keys and renumbers them. '> As an optional third parameter, you can pass a $compile_id. This is in the event that you want to compile different versions of the same template, such as having separate templates compiled for different languages. Another use for $compile_id is when you use more than one $template_dir but only one $compile_dir. Set a separate $compile_id for each $template_dir, otherwise templates of the same name will overwrite each other. You can also set the $compile_id variable once instead of passing this to each call to this function. '>