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# isset

(PHP 3, PHP 4 )

isset -- Determine whether a variable is set

## Description

bool isset ( mixed var [, mixed var [, ...]])

> Note: isset() is a language construct.

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.

> 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.

    <?php

    $var = '';

    // This will evaluate to &true; so the text will be printed.
    if (isset($var)) {
        print "This var is set set so I will print.";
    }

    // In the next examples we'll use var_dump to output
    // the return value of isset().

    $a = "test";
    $b = "anothertest";

    var_dump( isset($a) );      // TRUE
    var_dump( isset ($a, $b) ); // TRUE

    unset ($a);

    var_dump( isset ($a) );     // FALSE
    var_dump( isset ($a, $b) ); // FALSE

    $foo = NULL;
    var_dump( isset ($foo) );   // FALSE

    ?>

This also work for elements in arrays:

    <?php

    $a = array ('test' => 1, 'hello' => NULL);

    var_dump( isset ($a['test']) );            // TRUE
    var_dump( isset ($a['foo']) );             // FALSE
    var_dump( isset ($a['hello']) );           // FALSE

    // The key 'hello' equals NULL so is considered unset
    // If you want to check for NULL key values then try:
    var_dump( array_key_exists('hello', $a) ); // TRUE

    ?>

See also empty(), unset(), defined(), array_key_exists() and the error control @ operator.

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