PHP JSON响应以字符串值开头

I have some code which is quite simple, it looks to query an API using CURL and return the json response.

Here is the code:

<?php

...[VARIABLES]...

$ch = curl_init($url);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, $header);
curl_setopt($ch, CURLOPT_PROXY, $proxy);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);

$result = curl_exec($ch);
var_dump($result);

?>

Here is the response:

string(310) "{"totalPages":0,"firstPage":true,"lastPage":true,"numberOfElements":0,"number":0,"totalElements":0,"columns":{"columnIds":["metrics/visits:::0","metrics/visitors:::2","metrics/timespentvisit:::4"]},"summaryData":{"totals":[2740.0,1384.0,241.4753313696613]},"oberonRequestXML":[null],"oberonResponseXML":[null]}"

I'm struggling to understand why the string(310) is being shown at the front of the json response, since I am expecting a JSON response only.

use echo to print response

<?php

    ...[VARIABLES]...

    $ch = curl_init($url);
    curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
    curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_HTTPHEADER, $header);
    curl_setopt($ch, CURLOPT_PROXY, $proxy);
    curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);

    $result = curl_exec($ch);
    echo $result;

    ?>