What is the exists() function in php? There's array_key_exists(), class_exists(), dba_exists(), dbmexists(), file_exists(), function_exists(), interface_exists(), method_exists(), phar_offsetexists(), and property_exists().

Others have already mentioned empty (just test for (! $var)) and mysql (DBD::mysql and DBI), but mail's a tricky one, as there are lots of different mail modules in CPAN. The question's been asked many, many times on here ... but typically, they mention part of their requirements ... MIME, message size, quantity. I don't know if there's a nice list of what the features are of each of 'em.