curl -q -k --cert client-2048.crt --key client-2048.key https://identitysso.betfair.com/api/certlogin -d "username=testuser&password=testpassword" -H "X-Application: curlCommandLineTest"

Response should be

{"sessionToken":"Zx8i4oigut5nc+l4L8qFb0DSxG+mwLn2t0AMGFxjrMJI=","loginStatus":"SUCCESS"}

$ch = curl_init();

curl_setopt($ch, CURLOPT_HTTPHEADER, array('X-Application: $applicationkey'));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_URL, "https://identitysso.betfair.com/api/certlogin");

    $fields = array('username' => urlencode($username),'password' => urlencode($password));
    foreach($fields as $key=>$value) { $fields_string .= $key.'='.$value.'&'; }
    rtrim($fields_string, '&');

curl_setopt($ch,CURLOPT_POST, count($fields));
curl_setopt($ch,CURLOPT_POSTFIELDS, $fields_string);

$data = curl_exec($ch);
curl_close($ch);