<?php

$endPoint = "https://www.freelancer.com/api/projects/0.1/bids/";

$vars = array("project_id"=>18171040,"bidder_id"=> 14541573,"amount"=>18,"period"=>10,"milestone_percentage"=>100);
// $varString = http_build_query($vars);
// echo $varString;
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $endPoint);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS,$vars);  //Post Fields
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
// curl_setopt($ch, CURLOPT_HEADER, true);

$headers = [
    'freelancer-oauth-v1: [code_here]',
    'content-type: "multipart/form-data"'
];

curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);

$server_output = curl_exec ($ch);

curl_close ($ch);

print_r (json_decode($server_output, true)) ;