PHP中curl如何设置header？

<?php

$header  = array(
  'x-api-key:'.'b8602c0361111415a221759cdeb9e636',
  'Content-Type:'.'application/x-www-form-urlencoded; charset=UTF-8'
);

/**
 * @param $url
 * @param null $data
 * @return bool|string
 */
public function http_request($url, $data = null, $header = null){
 
    $curl = curl_init();
    if(!empty($header)){
        curl_setopt($curl, CURLOPT_HTTPHEADER, $header);
        curl_setopt($curl, CURLOPT_HEADER, 0);//返回response头部信息
    }
 
    curl_setopt($curl, CURLOPT_URL, $url);
    curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, false);
    curl_setopt($curl, CURLOPT_SSL_VERIFYHOST, false);
    if (!empty($data)) {
        curl_setopt($curl, CURLOPT_HTTPGET, 1);
        curl_setopt($curl, CURLOPT_POSTFIELDS,http_build_query($data));
    }
 
    curl_setopt($curl, CURLOPT_RETURNTRANSFER, TRUE);
    $output = curl_exec($curl);
    curl_close($curl);
    return $output;
}