The request is sent in ajax and cannot be received in the php background

Print is always blank, but the frontend has sent the request and passed the parameter.

Print is always blank, but the frontend has sent the request and passed the parameter.

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Print is always blank, but the frontend has sent the request and passed the parameter.

I did not see your front-end Processing

1. First, ajax datatype is not enclosed in quotation marks

2. I think you can first var_dump ($ _ POST); To see what upstream parameters are and whether there are values, debug one first

In the first line of this method, simply echo something to see if the ajax URL is correct. If there is output, the URL address is correct.
The var_dump () result should also be displayed.

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