If you want to verify if an email address actually exist and not just that it has the correct format and have access to the command line, you could use the exec() function in PHP to trigger a bash script and use it to find the mx records for the domain:
$response_str = exec("nslookup -q=mx example.com");
Then initiate a telnet session to the mail server(s) pointed out by the MX records:
exec("telnet mx1.mail.example.com 25");
Then try to assign a recipient, and you'll receive a response telling you if the address is valid or not.
exec("rcpt to: <firstname.lastname@example.org>");