How can the use of PHP functions like strtolower() and isset() be optimized to improve the overall performance and reliability of a script?

Using PHP functions like strtolower() and isset() in a script can impact performance if used inefficiently. To optimize their use, it is recommended to store the result of these functions in variables and reuse them instead of calling the functions multiple times. This can help reduce the number of function calls and improve the overall performance and reliability of the script.

// Inefficient use of strtolower() and isset()
if (isset($_POST['username']) && strtolower($_POST['username']) == 'admin') {
    // Do something
}

// Optimized use of strtolower() and isset()
$username = isset($_POST['username']) ? $_POST['username'] : '';
$usernameLower = strtolower($username);

if ($usernameLower == 'admin') {
    // Do something
}