JSON - pegar valor

Não tenho muito conhecimento em PHP. Como eu faço no json abaixo para retornar o "Subject" e o item "multipart/alternative"

[
[
"Received",
"by luna.mailgun.net with SMTP mgrt 8734663311733; Fri, 03 May 2013 18:26:27 +0000"
],
[
"Content-Type",
[
"multipart/alternative",
{
"boundary": "eb663d73ae0a4d6c9153cc0aec8b7520"
}
]
],
[
"Mime-Version",
"1.0"
],
[
"Subject",
"Test deliver webhook"
],
[
"From",
"Bob <bob@>"
],
[
"To",
"Alice <alice@example.com>"
],
[
"Message-Id",
"<20130503182626.18666.16540@mkt>"
],
[
"X-Mailgun-Variables",
"{\"my_var_1\": \"Mailgun Variable #1\", \"my-var-2\": \"awesome\"}"
],
[
"Date",
"Fri, 03 May 2013 18:26:27 +0000"
],
[
"Sender",
"bob@mkt."
]
]