The solution is simple. The only think you have to do, is to pass the array to the php function json_encode.

This will convert automatically your array to JSON.

The same way, you could also convert PHP Objects to JSON.

Finally, there's also another function of PHP with the name json_decode that does the opposite. If you have a JSON string, you can convert it to PHP Array or Object.

The only difference, is that when you convert JSON objects, in PHP you get stdClass instances.

<?php

$array = [
    [
        "name" => "Ville1",
        "description" => "adresse1",
        "lng" => -10.35,
        "lat" => 29.1833
    ],
    [
        "name" => "Ville2",
        "description" => "description2",
        "lng" => 12.61667,
        "lat" => 38.3833
    ]
];

$jsonStructure = json_encode($array);

?>
<script>
var locations = <?php echo $jsonStructure; ?>;

// The following line should print in your browser console 
// the word: Ville1
console.log(locations[0].name);
</script>

Please update your PHP Code accordingly. It should be working for you.

$array = [
    [
        "name" => "Ville1",
        "description" => "adresse1",
        "lng" => -10.35,
        "lat" => 29.1833
    ],
    [
        "name" => "Ville2",
        "description" => "description2",
        "lng" => 12.61667,
        "lat" => 38.3833
    ]
];
foreach ($array as $keys => $value ) {
    $object->{$keys} = $value;
}
$jsonStructure = json_encode($object); 
echo $jsonStructure;

The output should be like this :

{
  "0": {
    "name": "Ville1",
    "description": "adresse1",
    "lng": -10.35,
    "lat": 29.1833
  },
  "1": {
    "name": "Ville2",
    "description": "description2",
    "lng": 12.61667,
    "lat": 38.3833
  }
}

Now you can use this $jsonStructure to your javascript.