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# isset

(unknown)

isset -- Determine whether a variable is set

## Description

``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

See also empty() and unset().

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