create multidimensional json array from a json in php

Try this, use $map to store key city-zone, then transform to array to get the result

<?php

$val = [
    [
        'city' => 'xx',
        'zone' => 'yy',
        'person_on_zone' => 'p1'
    ],
    [
        'city' => 'xx',
        'zone' => 'yy',
        'person_on_zone' => 'p2'
    ],
    [
        'city' => 'xx',
        'zone' => 'yy',
        'person_on_zone' => 'p3'
    ],
    [
        'city' => 'xx',
        'zone' => 'ww',
        'person_on_zone' => 'p1'
    ]
];

$map = [];
foreach ($val as $v) {
    $key = $v['city'] . $v['zone'];
    if (!isset($map[$key])) {
        $map[$key] = [
            'city' => $v['city'],
            'zone' => $v['zone'],
            'person_info' => []
        ];
    }

    $map[$key]['person_info'][] = $v['person_on_zone'];
}

print_r(array_values($map));

Correct Code

  foreach ($val as $v){
                    $new_array['city'] = $v['city'];
                    $new_array['zone'] = $v['zone'];
                    foreach ($val as $v2){
                        $new_array['person_info'] = $v2['person_on_zone'];
                    }
                }
                json_encode($new_array);

try this:

first decode json array then use foreach loop with key

$val = json_decode($val);
foreach ($val as $v){
         $new_array['city'] = $v->city;
         $new_array['zone'] = $v->zone;
         foreach ($val as $key=>$v2){
         $new_array[$key]['person_info'] = $v->person_on_zone;
                    }
                }
 print_r($new_array);

I think you make a mistake around composing the json. I tried with your code and i found following is the correct way to do.. Remember single quote(') is not valid in json string that is being used to define key value pair in php

// I think your code has json string and i convert it into array of objects(stdClass) 
// and if your code has array then keep you code intact but need to change the notation of 
// objects to associative array.

// first thing first decode json string
$values =  json_decode('[
{ "city":"xx",
  "zone":"yy",
  "person_on_zone":"p1"
},
{
 "city":"xx",
 "zone":"yy",
 "person_on_zone":"p2"
},
{
 "city":"xx",
 "zone":"yy",
 "person_on_zone":"p3"
},
{
 "city":"xx",
 "zone":"ww",
 "person_on_zone":"p1"
}]');

// Now declare new values as array
$new_values = [];
// walk through each object
foreach ($values as $v){
    // $v becomes an object of stdClass here
    // $data is a temporary array
    $data['city'] = $v->city;
    $data['zone'] = $v->zone;
    $data['person_info'] = [];
    foreach ($values as $v2){
        if(!in_array($data['person_info'],['person_on_zone' => $v2->person_on_zone]))
        {
            // compare if zones are same or not
            if($v->zone == $v2->zone)
            {
                // push to the array instead of assiging
                array_push($data['person_info'], ['person_on_zone' => $v2->person_on_zone]);
            }
        }

    }
    // make array unique 
    $data['person_info'] = array_map("unserialize", array_unique(array_map("serialize", $data['person_info'])));
    array_push($new_values, $data);
}
// make array unique in mutidimensional array
$new_values = array_map("unserialize", array_unique(array_map("serialize", $new_values)));
echo '<pre>';
print_r($new_values);
echo '</pre>';

One simple foreach would do,

$result = [];
foreach ($arr as $key => $value) {
    //created unique combination of city and zone as key
    $result[$value['city'] . $value['zone']]['city']          = $value['city'];
    $result[$value['city'] . $value['zone']]['zone']          = $value['zone'];
    $result[$value['city'] . $value['zone']]['person_info'][] = ['person_on_zone' => $value['person_on_zone']];
}
echo json_encode(array_values($result));die;

array_values — Return all the values of an array

Working demo.