Returns TRUE if var exists; FALSE otherwise.
If a variable has been unset with unset(), it will no longer be set. isset() will return FALSE if testing a variable that has been set to NULL. Also note that a NULL byte ("\0") is not equivalent to the PHP NULL constant.
Warning: isset() only works with variables as passing anything else will result in a parse error. For checking if constants are set use the defined() function.
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This also work for elements in arrays:
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注: 由於這是一個語言結構、而不是函數,所以不能以變數函數的方式來呼叫。
See also empty(), unset(), defined(), the type comparison tables, array_key_exists(), and the error control @ operator.