<label>First Name <span>*</span></label>[text* your-firstname ]
<label>Last Name <span>*</span></label>[text* your-lastname ]
<label>Email <span>*</span></label>[email* your-email ]
<label>Phone</label>[text your-phone ]
<label>Message <span>*</span></label>[textarea* your-message ]

add_action( 'wpcf7_before_send_mail', 'my_conversion' );
function my_conversion( $cf7 )
{
  $email = $cf7->posted_data["your-email"];
  $first_name  = $cf7->posted_data["your-firstname"];
  $last_name  = $cf7->posted_data["your-lastname"];
  $phone = $cf7->posted_data["your-phone"];
  $company = $cf7->posted_data["your-company"];
  $message  = $cf7->posted_data["your-message"];
  $lead_source = $cf7->title;
 
  $post_items[] = 'oid=<YOU_SALESFORCE_OID>';
  $post_items[] = 'first_name=' . $first_name;
  $post_items[] = 'last_name=' . $last_name;
  $post_items[] = 'email=' . $email;
  $post_items[] = 'phone=' . $phone;
  $post_items[] = 'company=' . $company;
  $post_items[] = 'description=' . $message;
  $post_items[] = 'lead_source=' . $lead_source;
 
  if(!empty($first_name) && !empty($last_name) && !empty($email) )
  {
    $post_string = implode ('&', $post_items);
    // Create a new cURL resource
    $ch = curl_init();
 
    if (curl_error($ch) != "")
    {
      // error handling
    }
 
    $con_url = 'https://www.salesforce.com/servlet/servlet.WebToLead?encoding=UTF-8';
    curl_setopt($ch, CURLOPT_URL, $con_url);
    // Set the method to POST
    curl_setopt($ch, CURLOPT_POST, 1);
    // Pass POST data
    curl_setopt( $ch, CURLOPT_POSTFIELDS, $post_string);
    curl_exec($ch); // Post to Salesforce
    curl_close($ch); // close cURL resource
  }
}