How does php automatically update foreground data?

How does php automatically update foreground data? for example, if the administrator recharges the customer 100 yuan in the background and the customer does not refresh the page on the foreground, the amount of the user can be seen to change. The foreground uses ajax, there are two methods: AJAX asynchronous refresh and server push, but server push consumes server resources, and AJAX asynchronous refresh is better. how can aj php automatically update foreground data?
For example, if the administrator recharges the customer 100 yuan at the backend and the customer does not refresh the page at the front end, he or she can see that his or her amount has changed.

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Use ajax at the front end
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There are two methods: AJAX asynchronous refresh and server push, but server push consumes server resources, and AJAX asynchronous refresh is better.
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Ajax asynchronous update