You can't access the superglobals via variable variables (as you did with expressions like ${'_' . $a}) as stated in the manual Variable variables:
Warning Please note that variable variables cannot be used with PHP's Superglobal arrays within functions or class methods. The variable $this is also a special variable that cannot be referenced dynamically.
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您无法通过变量（如
$ {'_''。$ a}）通过变量访问超级全局，如手册变量变量：You can't access the superglobals via variable variables (as you did with expressions like
${'_' . $a}) as stated in the manual Variable variables: