JSON array values into variables (Linkedin PHP library)

I'm a noob at APIs and JSON that would appreciate any help on the following. I'm using the PHP Linkedin Library to run People Queries.

Here's the relevant code:

<?php
            $OBJ_linkedin->setResponseFormat(LINKEDIN::_RESPONSE_JSON);
            $keywords = (isset($_GET['keywords'])) ? $_GET['keywords'] : "Marketing";
            ?>
            <form action="<?php echo $_SERVER['PHP_SELF'];?>#peopleSearch" method="get">
                Search by Keywords: <input type="text" value="<?php echo $keywords?>" name="keywords" /><input type="submit" value="Search" />
            </form>
            <?php 
            $query    = '?sort=distance&current-company=true&keywords='.$keywords;
            $response = $OBJ_linkedin->searchPeople($query);

            if($response['success'] === TRUE) {


 echo "<pre>" . print_r($response['linkedin'], TRUE) . "</pre>";
            } else {
              // request failed
              echo "Error retrieving people search results:<br /><br />RESPONSE:<br /><br /><pre>" . print_r($response) . "</pre>";                
            }
          } else {
            // user isn't connected
            ?>

Here's an excerpt of the output I get

{"people": {
  "_count": 10,
  "_start": 0,
  "_total": 11,
  "values": [
    {
      "firstName": "Peter",
      "headline": "Frontend Engineer at Lot18",
      "id": "kYZ3B2hHYH",
      "lastName": "Welch",
      "pictureUrl": "http://m.c.lnkd.licdn.com/mpr/mprx/0_e0hbvSXvhiSoTO2PERiqvfLV850d342PoOq4vakwx8IJOyR1XJrwRmr5mIx9C0DxWpGMsWVjBZEQ",
      "relationToViewer": {"distance": 3}
    },

  ]
}}

I'd like to capture the fields like "firstname" and "pictureUrl" into variables that I can use elsewhere. E.g.

<img src="<?php echo $picture-url; ?>" />

How would I go about doing that? I've spent days searching/trying to figure this out and still no luck. Any help is much appreciated!