<?php
###################
####   USAGE   ####
###################
$multidimensional_array = [[1,2, [6]], [5], 'hasan', null, -20, 4];
$flattened_array = array();

echo "<pre>";

	echo "<br>Flattened array using global variable:<br>";
	flatten_array($multidimensional_array);
	print_r($flattened_array);

	echo "<br>Flattened array by reference:<br>";
	$flattened_array_using_reference = flatten_array_by_reference($multidimensional_array);
	print_r($flattened_array_using_reference);

echo "</pre>";


###################
####  METHODS  ####
###################

function flatten_array($array) {
	# Method to flatten a multidimensional array to one dimension.
	# Using a global variable
	#
	# $array = an array to convert into one dimension

	if(is_array($array)){
		foreach($array as $item) {
			if(is_array($item)) {
				flatten_array($item);
			} else {
				$GLOBALS['flattened_array'][] = $item;
			}
		}
	}
}

function flatten_array_by_reference($array, &$out = array()) {
    # Method to flatten a multidimensional array to one dimension.
    # using, array passing by reference
    #
    # $array - an array to convert into one dimension
    # $out - internal used argument for recursion

    foreach($array as $key => $child){
        if(is_array($child))
            $out = flatten_array_by_reference($child, $out);
        else
            $out[] = $child;
    }
    return $out;
}
?>