As a web-focused language, it is common in PHP to process user-supplied data. Such data needs to be tested before it is used to confirm that it exists and has a valid value. PHP provides a number of built-in constructs that can be used for this purpose.
if the variable exists and has been assigned a value other than null.
$a = 10;
isset($a); // true
$a = null;
isset($a); // false
The empty construct
checks whether the specified variable has an empty value—such as null, 0, false, or an empty string—and returns true if ...