exec nodejs script from php

i have problem with a nodejs script used from php using exec() in a loop . the problem is that the script is executed only once then just errors. here is the php code :

for ($i=0; $i <= ((mysqli_num_rows($repp)/50)-1); $i++) 
{   

  $req = "select * from items  limit ".$id.",50"; 
  $rep = mysqli_query($link,$req);
  $names = "";
  $id = $id + 50;

  while($donne = mysqli_fetch_array($rep))
    {
            $names = $names.$donne["item_name"].',';
    }
  $resultat = '"'.rtrim($names, ",").'"';
  exec("node node_opskins.js ".$resultat,$out);
  $result =implode($out);
  $a = str_replace("'", '"', $result);
  $a = preg_replace('/([\w\d]+): /', '"$1": ', $a);
  foreach ( json_decode($a,true) as $key => $value) 
  {
    echo $key . " : " . $value['market_price'] . "<br/>";
    $req = "update items set opskins_suggested = '".($value['market_price']/100)."' where item_name = '".addslashes($key)."'";
        mysqli_query($link,$req);
 }
 sleep(10);
}

and here is the Nodejs script :

var OPSkinsAPI = require('@opskins/api');
var opskins = new OPSkinsAPI(my opskins api key);
var item = process.argv[2].split(",");
opskins.getSuggestedPrices(730, item, function(err,price){ 


    console.log(price);

});

via ishak hadje