isset Determine if a variable is set and is not &null; &reftitle.description; boolisset mixedvar mixed... Determine if a variable is set and is not &null;. 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 character ("\0") is not equivalent to the PHP &null; constant. If multiple parameters are supplied then isset will return &true; only if all of the parameters are set. Evaluation goes from left to right and stops as soon as an unset variable is encountered. &reftitle.parameters; var The variable to be checked. ... Another variable ... &reftitle.returnvalues; Returns &true; if var exists and has value other than &null;. &false; otherwise. &reftitle.changelog; &Version; &Description; 5.4.0 Checking non-numeric offsets of strings now returns &false;. &reftitle.examples; ]]> This also work for elements in arrays: 1, 'hello' => NULL, 'pie' => array('a' => 'apple')); 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', $a)); // TRUE // Checking deeper array values var_dump(isset($a['pie']['a'])); // TRUE var_dump(isset($a['pie']['b'])); // FALSE var_dump(isset($a['cake']['a']['b'])); // FALSE ?> ]]> PHP 5.4 changes how isset behaves when passed string offsets. ]]> &example.outputs.53; &example.outputs.54; &reftitle.notes; 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. ¬e.language-construct; When using isset on inaccessible object properties, the __isset() overloading method will be called, if declared. &reftitle.seealso; empty __isset() unset defined the type comparison tables array_key_exists is_null the error control @ operator