Extracting specific value from JSON string with PHP

Question

So I have what appears to be a pretty simple problem, though for the life of me with my lack of coding knowledge I can't figure it out. Excuse any misnomers! So I have a URL heading to a page which contains a signed request with a JSON string in it (it's a Facebook page). I need to retrieve that JSON string and then extract out just a specific part of it. I have no control over the formatting of the URL at the moment.

On the page that it goes to I have the following code:

<?php
$signed_request = $_REQUEST['signed_request']; // Get the POST signed_request variable.

if(isset($signed_request)) // Determine if signed_request is blank.
{
 $pre = explode('.',$signed_request); // Get the part of the signed_request we need.
 $json = base64_decode($pre['1']); // Base64 Decode signed_request making it JSON.
 $obj = json_decode($json,true); // Split the JSON into arrays.

 echo $obj['app_data'];
}
 else
{
die('No signed request avaliable.'); //If there is no signed_request, stop processing script.
}
?>

That works fine, and echo $obj['app_data']; prints: {q:"id_src=abc123456789",}

To me, that in itself is a JSON string, so I thought I could run a json_decode on that and then print out id_src=abc123456789. I tried that like this:

$appdata = $obj['app_data'];
$idcode = json_decode($appdata,true);

Followed by echo $idcode['q'], but that just prints a single curly bracket. I tried various variations, removal of ' ' marks, removal of true, etc.

My end goal is to just extract the abc123456789 value, not the id_src part. I figured it was probably faster to just ask since I'm unlikely to figure it out in the next few days.

Thanks for the help all!

EDIT: Snippet of $json added. echo $json; prints the following

{"algorithm":"HMAC-SHA256","app_data":"{q:\"id_src=abc123456789\",}","issued_at":1333500860,"page":{"id":"380641958713853","liked":false,"admin":true},"user":{"country":"nz","locale":"en_GB","age":{"min":21}}}

Would it be the comma after the id_src value that is causing problems?

Can you post a snippet of the returned JSON after you base64_decode() it? It's possible your JSON has become corrupted somewhere in the process of fetching it, but we can't know for sure without a snippet of it. — Bojangles
– Bojangles, Commented Apr 4, 2012 at 0:46

Igor Escobar · Accepted Answer · 2012-04-04 00:56:26Z

2

You can extract what you want like this:

preg_match('/"\\w+?=(\\w+?)"/', $obj['app_data'], $matches);

print_r($matches); // Array ( [0] => "id_src=abc123456789" [1] => abc123456789 )

answered Apr 4, 2012 at 0:56

Igor Escobar

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2 Comments

Dave Over a year ago

My hero! Thanks, print_r($matches[1]) prints exactly what I want.

Igor Escobar Over a year ago

Thumbs up the answer if it helped you :D

SeanCannon · Accepted Answer · 2012-04-04 00:46:53Z

0

{q:"id_src=abc123456789",} is not valid JSON. Try {"q":"id_src=abc123456789"}

answered Apr 4, 2012 at 0:46

SeanCannon

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1 Comment

Dave Over a year ago

Hmmm that's what the json_decode function prints out, and that comes from the decoding of the signed_request passed into the iFrame. So this could mean there is an error in the way app_data is appended to the URL - is there any way to extract the id_src value as is?

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