Q: How to use the wrapper?

Answer:

<?php
 require 'PWM.php';
 $pwm = new PWM();
 $pwm->SetChan('all'); //set the channels
 /* The CHANNEL can be: 0 - 15 to control an individual channel
    The CHANNEL can be: all to control all channels at once
    
   Optional parameters: frequency If not specified, default of 50 Hz is used
   Frequency range is 24 Hz to 1526 Hz
   Adds a delay in the PWM signal, can be an integer or floating point number between 0 and 100, 0% delay by default
 */
 $data = $pwm->SetDuty(1.25); // sets duty_cycle 1.25 on all channel's
 echo explode(':',$data[1])[1]; // Parse just the responce
 
 ?>

Q: Where do I get it?

Answer: Right here!

The Gist Link

<?php
/**
* @ Author: Chris McCaslin
* @ Date: 5/25/2016
* @ Disc: Hardware pwm wrapper!
*/
  class PWM
  {
    private $channel;
    private $duty_cycle;
    private $frequency;
    private $delay;
    
    // Need to intilize pwm
    function __constructor()
    {
      $this->consoleCmd("ubus call i2c_exp pwm-exp '{\"command\":\"init\"}'");
    }
    // The SetPeriod function https://github.com/OnionIoT/i2c-exp-driver#set-period-command
    public function SetPeriod($pulse, $period)
    {
      return $this->ConsoleCmd("ubus call i2c_exp pwm-exp '{\"command\":\"set-period\", \"params\":{\"channel\":$this->channel, \"pulse\":$pulse, \"periods\":$period}}'");
    }
    
    public function SetChan($chn)
    {
      $this->channel = $chn;
    }
    // The SetDuty Command https://github.com/OnionIoT/i2c-exp-driver#set-command
    public function SetDuty($dtyCycl)
    {
      if($dtyCycl < 0.0000)
      {
        return "The DUTY CYCLE can be an integer or floating point number between 0 and 100";
      }
      elseif ($dtyCycl >100.0000)
      {
        return "The DUTY CYCLE can be an integer or floating point number between 0 and 100";
      }
      else
      {
        $this->duty_cycle = (float)$dtyCycl;
      }
      return $this->ConsoleCmd("ubus call i2c_exp pwm-exp '{\"command\":\"set\", \"params\":{\"channel\":\"$this->channel\",\"duty\":\"$this->duty_cycle\"} }'");
    }
    // The SetFreq Command Optional parameter
    public function SetFreq($dtyCycl, $freq)
    {
      if($dtyCycl < 0.0000)
      {
        return "The DUTY CYCLE can be an integer or floating point number between 0 and 100";
      }
      elseif ($dtyCycl >100.0000)
      {
        return "The DUTY CYCLE can be an integer or floating point number between 0 and 100";
      }
      else
      {
        $this->duty_cycle = (float)$dtyCycl;
      }
      if($freq < 24)
      {
        return "Frequency range is 24 Hz to 1526 Hz";
      }
      else if($freq > 1526)
      {
        return "Frequency range is 24 Hz to 1526 Hz";
      }
      else
      {
        $this->frequency = $freq;
      }
      return $this->ConsoleCmd("ubus call i2c_exp pwm-exp '{\"command\":\"set\", \"params\":{\"channel\":\"$this->channel\", \"duty\":\"$this->duty_cycle\", \"frequency\":\"$this->frequency\"} }'");
    }
    // The SetDelay Command Optional parameter
    public function SetDelay($dtyCycl, $freq, $delay)
    {
      if($dtyCycl < 0.0000)
      {
        return "The DUTY CYCLE can be an integer or floating point number between 0 and 100";
      }
      elseif ($dtyCycl >100.0000)
      {
        return "The DUTY CYCLE can be an integer or floating point number between 0 and 100";
      }
      else
      {
        $this->duty_cycle = (float)$dtyCycl;
      }
      if($freq < 24)
      {
        return "Frequency range is 24 Hz to 1526 Hz";
      }
      else if($freq > 1526)
      {
        return "Frequency range is 24 Hz to 1526 Hz";
      }
      else
      {
        $this->frequency = $freq;
      }
      if($delay < 0.0000)
      {
        return "The DUTY CYCLE can be an integer or floating point number between 0 and 100";
      }
      elseif ($delay >100.0000)
      {
        return "The DUTY CYCLE can be an integer or floating point number between 0 and 100";
      }
      else
      {
        $this->delay = (float)$delay;
      }
      return $this->ConsoleCmd("ubus call i2c_exp pwm-exp '{\"command\":\"set\", \"params\":{\"channel\":\"$this->channel\", \"duty\":\"$this->duty_cycle\", \"frequency\":\"$this->frequency,  \"delay\":\"$this->delay\"} }'");
    }
    // The Sleep Command https://github.com/OnionIoT/i2c-exp-driver#sleep-command
    public function Sleep()
    {
      return $this->ConsoleCmd("ubus call i2c_exp pwm-exp '{\"command\":\"sleep\"}'");
    }
    // A method so we can interface with console easy.
    private function ConsoleCmd($cmdToSend)
    {
      exec("$cmdToSend 2>&1", $output);
      return $output;
    }
  }
 ?>

the pull Request

I had to google what PWM means