When do I need to add a colon to the method before the thinkphp template?

For example, I want to link to the homepage in the navigation:
Written in this way: "{U (' Index/index ')}"
cannot be linked correctly to the homepage.
Written like this: "{: U (' Index/index ')}"
(That is, the U method is preceded by a colon)

But I'm in another place, in a form form:

U method before adding a colon no problem!

Why is that?

Reply content:

For example, I want to link to the homepage in the navigation:
Written in this way: "{U (' Index/index ')}"
cannot be linked correctly to the homepage.
Written like this: "{: U (' Index/index ')}"
(That is, the U method is preceded by a colon)

But I'm in another place, in a form form:

U method before adding a colon no problem!

Why is that?

1, U() is the function that generates the URL in the thinkphp
2, in the template, the use of {:U('home/index')} this method means:
3, if only write {U('home/index')} should be understood as invalid (also may be executed but not echo), not specifically tested, and LZ said in the form without a colon is not a problem may be because this write invalid, no output anything, so it becomes , In the form tag, the action is empty and is submitted to the current page by default. So..

Link description

