Usage of php isset function

isset function checks whether the variable is set.

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

Return value:

If the variable does not exist, return FALSE Also returns FALSE if the variable exists and its value is NULL If the variable exists and the value is not NULL, TRUE is returned When checking multiple variables at the same time, TRUE is returned only when each single item meets the previous requirement, otherwise the result is FALSE. If a variable has been freed using unset(), it will no longer be isset(). If you use isset() to test a variable that is set to NULL, it will return FALSE. Also note that a NULL byte ("

Warning: isset() can only be used with variables, as passing any other parameters will cause a parsing error. If you want to detect whether a constant has been set, use the defined() function.

Example 1:

$var = '';
if (isset($var)) {
 print "This var is set set so I will print.";
}// In the following example , we will use the var_dump function 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
?>

Example 2:

$a = array ('test' => 1, 'hello' => NULL);
var_dump( isset ($a['test') ); // TRUE
var_dump( isset ($a['foo') ); // FALSE
var_dump( isset ($a['hello') ); // FALSE
// 'hello' is equal to NULL, so it is considered unassigned.
// If you want to detect NULL key values, you can try the method below.
var_dump( array_key_exists('hello', $a) ); // TRUE
?>