<?php
$jsonString1 = '[{
        "id": "17",
        "pf_label": "Gender"
    },
    {
        "id": "18",
        "pf_label": "Age"
    },
    {
        "id": "12",
        "pf_label": "Address Line"
    }
]';


$jsonString2 = '[{
    "": "",
    "17": "male",
    "18": "27"
}, {
    "": "",
    "17": "female",
    "18": "26",
    "12": "japan"
}]';


$array1 = json_decode($jsonString1, true);
$indexes = array();
foreach ($array1 as $element) {
    $indexes[$element['id']] = $element['pf_label'];
}


$array2 = json_decode($jsonString2, true);
foreach ($array2 as $element) {
    foreach ($element as $key => $value) {
        if ($key) {
            $singleElement[$indexes[$key]] = $value;
        } else {
            $singleElement[""] = "";
        }
    }
    $result[] = $singleElement;
}


var_dump($result);

array(2) {
  [0]=>
  array(3) {
    [""]=>
    string(0) ""
    ["Gender"]=>
    string(4) "male"
    ["Age"]=>
    string(2) "27"
  }
  [1]=>
  array(4) {
    [""]=>
    string(0) ""
    ["Gender"]=>
    string(6) "female"
    ["Age"]=>
    string(2) "26"
    ["Address Line"]=>
    string(5) "japan"
  }
}