paypal IPN get buyer transaction ID
$_POST['txn_id']

Array
(
    [mc_gross] => 7.00
    [protection_eligibility] => Ineligible
    [payer_id] => F6912JHUIIHA
    [tax] => 0.00
    [payment_date] => 10:14:55 Sep 11, 2011 PDT
    [payment_status] => Completed
    [charset] => windows-1252
    [first_name] => Name
    [mc_fee] => 2.08
    [notify_version] => 3.2
    [custom] =>
    [payer_status] => verified
    [business] => myemail@gmail.com
    [quantity] => 1
    [verify_sign] => 123232jh4i32u4u3h5n435i43u5455645
    [payer_email] => payermail@gmail.com
    [txn_id] => 123u4324324yuy4574
    [payment_type] => instant
    [btn_id] => 35428120
    [last_name] => lastname
    [receiver_email] => receiver@gmail.com
    [payment_fee] => 2.08
    [shipping_discount] => 0.00
    [insurance_amount] => 0.00
    [receiver_id] => OIUWDWUERWEU
    [txn_type] => web_accept
    [item_name] => Product name
    [discount] => 0.00
    [mc_currency] => USD
    [item_number] => PRODNum
    [residence_country] => SE
    [handling_amount] => 0.00
    [shipping_method] => Default
    [transaction_subject] => Product to sell
    [payment_gross] => 7.00
    [shipping] => 0.00
    [ipn_track_id] => ydedi23484rl4itm54oi
)