$ _ SERVER ['document _ URI '] what values can be returned? Who knows?

$ _ SERVER ['document _ URI '] what values can be returned? Who knows? if RUL is http: // 127.0.0.1/test. php $ a = $ _ SERVER ['document _ URI ']; print_r ($ a); why is this empty, what values do I expect t $ _ SERVER ['document _ URI '] to return? Who knows?
If RUL is http: // 127.0.0.1/test. php
$ A = $ _ SERVER ['document _ URI '];
Print_r ($ a); why is it empty? I thought I would get the test. php part.

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No SERVER variable $ _ SERVER ['document _ URI ']
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This server variable nginx has, apache does not, iis does not know. Use other common point variables.
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$ _ SERVER ['request _ URI '];
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$ _ SERVER ['request _ URI ']