boolean isset (mixed var)
Returns TRUE if var exists; FALSE otherwise.
If a variable has been unset with unset(), it will no longer be isset(). 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.
$a = "test";
echo isset ($a); // TRUE
unset ($a);
echo isset ($a); // FALSE
$foo = NULL;
print isset ($foo); // FALSE
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