isset() function in PHP is used to check if a variable is set and not 

Purpose: isset() checks if a variable exists and has a value other than NULL.

Syntex:

isset(variable, ....);

$name = "John";
$age = 30;

if (isset($name)) {
    echo "Name is set.\n";
} else {
    echo "Name is not set.\n";
}

Output:

Name is set.

Combining with Ternary Operator: isset() 

$var = isset($someVar) ? $someVar : 'default';

// This is equivalent to:
// $var = $someVar ?? 'default';