How can select fail to get the selected value?

Why can't the select value be selected? & Lt; METAcontenttexthtml; charsetgb2312http-equivContent-Type & gt; & lt ;? Php & nbsp; $ a $ _ GET [showtype]; & nbsp; echo & quot; & nbsp; & lt: how can I not GET the selected value for select?

$ A = $ _ GET [showtype];
Echo"

No.	Class	Score	Level	Course

";
Echo $;
?>

The above is the code. I want to get the value of $ a, but why is $ a blank? how can I change it? There is also onchange = what will be written here and the $ a value will be output after selection?

------ Solution --------------------
Assume that the file name is test. php.

$ A =! Empty ($ _ GET ["showtype"])? $ _ GET ["showtype"]: 0;

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------ Solution --------------------DiscussionAnd why not $ a =$ _ GET [showtype?