How to insert check box data into the database in the form

How do I insert the check box data in the form into the database? I made a check box in the form. The data in the selected check box is saved in an array, traversed the array, and each element is put together and saved in a string. For example, [select] basketball [] Badminton [] table tennis [select] selected Shot put: the basketball shot put is saved in the $ pre_u_have variable echo $ pre_u_have; // it can be displayed: in basketball shot put, how do I insert the check box data in the table form into the database?
I made a check box in the form. The data in the selected check box is saved in an array, traversed the array, and each element is put together and saved in a string. For example:
[Choice] Basketball
[] Badminton
[] Table tennis
【Option】 shot put
The selected: basketball shot put is saved in the $ pre_u_have variable.
Echo $ pre_u_have; // displayed: basketball shot put

I want to insert the form into the database. check box data is inserted into the u_have field.
The u_have field cannot match Form. u_have [] because it is a check box.
So I inserted a hidden domain
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In this example, $ pre_u_have is directly assigned to u_have. the insert operation is successful, but the field in the database displays $ pre_u_have (the variable name is treated as a string)
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This shows that coloum "u_have" does not allow NULL, which means data insertion into the database fails.
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Where is my current problem? I stayed up all night and the problem still could not be solved.

What we see on the Internet is:
$ _ POST ['pre _ u_have '];
This statement

The key is where to place this step?

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Print_r ($ _ POST ['tem ']);
?>
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Discussion

Print_r ($ _ POST ['tem ']);
?>

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Maybe this is not a problem.
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PUT the PHP code IN line 1st, followed by the for loop, NO PUT IN HTML