This scripts collects all arguments of the function consisting of primitives and arrays and then flattens into a one dimensional array.

ArrayFlatter.php

<?php
/**
*@author Ibrahim Samad
*@date 6/6/2017
*@description A simple class that flatten a multi-dimensional array into a single array 
*while taking care of null values.
*/

class ArrayFlatter
{
	/*
	*@param Array, primitives
	*Receives any number of arguments: primitives and arrays 
	*and then do a recursive walk incase of an array to collect all the 
	*individuals array elements onto a parent array
	*
	*/
	public function flatten()
	{

		$data = func_get_args()[0];

		return is_array($data) ? array_reduce($data, function($c, $a){
			return $this->format(array_merge($c, $this->flatten($a)));
		}, []) : [$data];

		
	}

	/*
	*@param Array
	*Convert null values to the string null.
	*/
	private function format($array)
	{
		return array_map(function($val){
			return $val != null ? $val : 'null';
		}, $array);
	}	
}


//Example usage

$obj = new ArrayFlatter;
echo '<pre>', print_r($obj->flatten(['a', ['b', 2], 3, null, [[4], ['c']]]), true),'</pre>';

//produces
/*Array
(
    [0] => a
    [1] => b
    [2] => 2
    [3] => 3
    [4] => null
    [5] => 4
    [6] => c
)*/