What is curl in php

cURL is a library that lets you make HTTP requests in PHP.
it is a way you can hit a URL from your code to get a html response from it. 
cURL means client URL which allows you to connect with other URLs and use their responses in your code

Remark:
The curl_exec command in PHP is a bridge to use curl from console. 
curl_exec makes it easy to quickly and easily do GET/POST requests, 
receive responses from other servers like JSON and download files.

Example:

$url = 'url_to_post';
    $data = array("first_name" => "First name","last_name" => "last name","email"=>"email@gmail.com","addresses" => array ("address1" => "some address" ,"city" => "city","country" => "CA", "first_name" =>  "Mother","last_name" =>  "Lastnameson","phone" => "555-1212", "province" => "ON", "zip" => "123 ABC" ) );
    $data_string = json_encode($data);
    $ch=curl_init($url);
    curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
    curl_setopt($ch, CURLOPT_POSTFIELDS, array("customer"=>$data_string));
    curl_setopt($ch, CURLOPT_HEADER, true);
    curl_setopt($ch, CURLOPT_HTTPHEADER,
               array('Content-Type:application/json',
                      'Content-Length: ' . strlen($data_string))
               );

    $result = curl_exec($ch);
    curl_close($ch);