isset Determine whether a variable is set boolisset mixedvar mixedvar ... 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. 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. ]]> This also work for elements in arrays: 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.