If you have previously enabled custom templates for core functions, then enable PHP templates without disabling the legacy templates, your legacy templates will override PHP templates where they exist. This only applies to enabled legacy templates – so if you have a custom template for your single details view, and want to use PHP templates to customize the grid view, you can do that. However, if you want to create a custom PHP template to replace that single view, the legacy template will need to be disabled.
1. Disable PHP Templating
The settings for legacy templating are only available with PHP templating disabled, so you’ll need to disable PHP templating at My Calendar > Settings > Display.
2. Browse to custom templates
Find the custom template settings at My Calendar > Design > Templates. Disable any legacy templates you don’t need.

3. Select the enabled template and disable.

When you click through on the template title, you can uncheck the ‘Use this template’ option, then save settings to disable the template.
4. Re-enable PHP templates
Once legacy templates are disabled, the related PHP templates will be used.