// Get array of all project users
$users = REDCap::getUsers();

// Print out a count of number of users
print "This project contains " . count($users) . " users.\n";

// Check if a specific user has access to this project
$user_to_look_for = "jon_williams";
if (in_array($user_to_look_for, $users)) {
    print "User $user_to_look_for has access to this project.";
} else {
    print "User $user_to_look_for does NOT have access to this project.";
}