Text file not written on the server

Question

I try to write a file on my server, but I don't understand what's happening : it's always empty (blank page). There is no error thrown. When I insert inside the loop a var_dump($lines), I see the data but at the moment an error appear

With the code below

$minLat = -3.0000; //41.34343606848294;
$maxLat = 22.0000; //57.844750992891;
$minLng = -0.0300; //-16.040039062500004;
$maxLng = 90.4200; //29.311523437500004;

$step = 0.1;
$k = 1;


$estimator = new KNearestNeighbors(9);
$estimator->train($dataset->getSamples(), $dataset->getTargets());
 $lines = [];

    for($lat=$minLat; $lat<$maxLat; $lat+=$step) {
      for($lng=$minLng; $lng<$maxLng; $lng+=$step) {
        $lines[] = sprintf('%s;%s;%s', $lat, $lng, $estimator->predict([[$lat, $lng]])[0]);
      }
    }

    var_dump($lines); ==> display info, but always empty

    $filename = '/var/www/test/php-ml/result_map.csv';

    //$content = implode(PHP_EOL, $lines);
    $content = implode( "\n", $lines );

file_put_contents($filename, $content);

the result (example) -------------

bool(true) ==> check if the file exist

array(1) { [0]=> string(41) "-3;-0.03;Apple 15 Inch MacBook Pro Laptop" }
array(2) { [0]=> string(41) "-3;-0.03;Apple 15 Inch MacBook Pro Laptop" [1]=> string(40) "-3;0.07;Apple 15 Inch MacBook Pro Laptop" }
array(3) { [0]=> string(41) "-3;-0.03;Apple 15 Inch MacBook Pro Laptop" [1]=> string(40) "-3;0.07;Apple 15 Inch MacBook Pro Laptop" [2]=> string(40) "-3;0.17;Apple 15 Inch MacBook Pro Laptop" }
array(4) { [0]=> string(41) "-3;-0.03;Apple 15 Inch MacBook Pro Laptop" [1]=> string(40) "-3;0.07;Apple 15 Inch MacBook Pro Laptop" [2]=> string(40) "-3;0.17;Apple 15 Inch MacBook Pro Laptop" [3]=> string(40) "-3;0.27;Apple 15 Inch MacBook Pro Laptop" }