注: 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.