Adding Mixpanel tracking for PHP

metrics_tracker.class.php

<?php
class MetricsTracker {
  
  public $token;
  public $host = 'http://api.mixpanel.com/';
  
  public function __construct($token_string) {
    $this->token = $token_string;
  }
  
  function track($event, $properties=array()) {
    $params = array(
      'event' => $event,
      'properties' => $properties
    );

    if (!isset($params['properties']['token'])){
      $params['properties']['token'] = $this->token;
    }
    $url = $this->host . 'track/?data=' . base64_encode(json_encode($params));
    
    exec("curl '" . $url . "' >/dev/null 2>&1 &"); 
  }
  
  function set($distinct_id, $properties=array(), $ip = "") {
    $params = array(
      '$distinct_id' => $distinct_id,
      '$set' => $properties,
      '$token' => $this->token
    );

  	if ($ip != "") {
			$params['$ip'] = $ip;
		}
  
    $url = $this->host . 'engage/?data=' . base64_encode(json_encode($params));
    
    exec("curl '" . $url . "' >/dev/null 2>&1 &"); 
  }
  
  function increment($distinct_id, $properties=array()) {
    $params = array(
      '$distinct_id' => $distinct_id,
      '$add' => $properties,
      '$token' => $this->token
    );
  
    $url = $this->host . 'engage/?data=' . base64_encode(json_encode($params));
    
    exec("curl '" . $url . "' >/dev/null 2>&1 &"); 
  }
  
  function track_charge($distinct_id, $amount) {
    $params = array(
      '$distinct_id' => $distinct_id,
      '$append' => array('$transactions' => array('$amount' => $amount, '$time' => substr(date("c"), 0, 19))),
      '$token' => $this->token
    );
  
    $url = $this->host . 'engage/?data=' . base64_encode(json_encode($params));
    
    exec("curl '" . $url . "' >/dev/null 2>&1 &"); 
  }
}
?>