从php / json多维数组中获取键/值，基于另一个值

I have struggled with this for a long time now. I have lost count of all the forum posts I have read and tried. Now I have reached the point where I have to turn to the experts. There are lots of questions and answers about arrays, but I have not been able to find one quite like mine. Or I have misunderstood them. If this is a duplicate question I apologize! Please point me in the right direction.

I want to dynamically feed a Google chart. Data comes from a web service, in json. I decode this to a php array. I am able to get data from the first ([SampleDate]) and second ([ReportedValue]) "level" and get them into a new array to feed the chart. But, I need to find other values corresponding to the other second level keys further down.

My first idea was to search (search parameter will come from a link from another page. But I will hopefully solve that later on.) for the right value (i.e [PropertyCode] => Ca), when found present the value for the corresponding key ([ReportedValue] => 2,15) and loop this to get all values (together with [Sampledate], for the chart). But I'm sure there are a lot of other ways to do what I am trying to do. The problem is I don't know how to proceed. If it's any help the complete code can be tested at PhpFiddle

I hope that you understand my question and are able to help me out.

Thanks in advance!

$stuff = json_decode($json, true);
print "<pre>";
print_r ($stuff);
$datum = array();
foreach ($stuff as $value){
$date = $value['SampleDate'];
    preg_match('~(\d+)\d{3}((?:\+|-)\d+)~', $date, $match);
    $dt = new DateTime('@' . $match[1]);
    $dt = new DateTime($dt->format('Y-m-d H:i:s') , new DateTimeZone('UTC'));
    $dt->setTimezone(new DateTimeZone('Europe/Stockholm'));
    $dates = $dt->format('Y');

    $valueChange = $value['ObservationValues'][0]['ReportedValue'];
    $number = str_replace(',', '.', $valueChange);

$datum[] = array("SampleDate" =>$dates, "ReportedValue" =>$number);
}
//counting the length of the array
$countArrayLength = count($datum);
/*print "<pre>";
print_r ($datum);
print "</pre>";*/
?>
<script type="text/javascript" src="https://www.google.com/jsapi"></script>
<script type="text/javascript">
google.load("visualization", "1", {packages:["corechart"]});
google.setOnLoadCallback(drawChart);

function drawChart() {

var data = new google.visualization.DataTable();
data.addColumn('string', 'Year');
data.addColumn('number', '/5cm');

data.addRows([
<?php
for($i=0;$i<$countArrayLength;$i++){
    echo "['" . $datum[$i]['SampleDate'] . "'," . $datum[$i]['ReportedValue'] . "],";
} 
?>
]);

var options = {
    title: 'Absorbans filtrerat (420 nm)',
    curveType: 'function',
    legend: { position: 'bottom' } 
};

    var chart = new google.visualization.LineChart(document.getElementById('curve_chart'));
chart.draw(data, options);
}
</script>

<div class="grid-container"> 
<div class="grid-100 grid-parent">
<div id="curve_chart" style="width: 250px; height: auto"></div>
</div>   

</div>

Piece of array:

Edit: I kept going at it and now I have an array that looks like this:

Array
(
[0] => Array
    (
        [0] => 2013
        [1] => 2.15
    )

[1] => Array
    (
        [2] => 2014
        [3] => 2.21
    )

[2] => Array
    (
        [4] => 2011
        [5] => 1.933
    )

[3] => Array
    (
        [6] => 2012
        [7] => 2.033
    )

[4] => Array
    (
        [8] => 2015
        [9] => 2.20
    )

)

For it to work with Google Charts I need to name the keys like this:

Array
(
[0] => Array
    (
        [SampleDate] => 2013
        [ReportedValue] => 2.15
    )

[1] => Array
    (
        [SampleDate] => 2014
        [ReportedValue] => 2.21
    )... and so on

Yet again I've spent many hours and tried everything I found even slightly in the same ballpark. With no luck. Please help!