Returns TRUE if var exists; FALSE otherwise.
If a variable has been unset with unset(), it will no longer be set. 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 ?> |
Note: Because this is a language construct and not a function, it cannot be called using variable functions
See also empty(), unset(), defined(), the type comparison tables, array_key_exists(), and the error control @ operator.